Why was this instance a good use case for ridicule?
As you admit, the question asker might very well have been genuinely curious. The answer to their question doesn't seem blindingly obvious to me (although I can certainly speculate about good reasons that grading isn't automated). I can't tell what you mean by "unacceptable" (perhaps there is some cultural taboo you're referring to here?). And it also doesn't seem "dangerously naive", which might apply where someone asking this question had decision making power in a critical situation, but clearly doesn't apply here.
By the very criteria you lay out, this doesn't seem like an instance in which ridicule was necessary or good. To me, it feels like a weak justification for bullying someone who thinks differently about something you obviously feel very strongly about.
I don't believe this was a case of bullying at all. It was a light ribbing. It also aligned with an archetype (of being able to engineer our way out of human problems) that's especially salient for some people on here, yet totally lost on others. The contradiction is funny, too, because it's classic human folly. Whether or not the commenter is anything like that really doesn't figure into the calculus of how necessary or good this instance of another commenter's snarky side comment was: there are no material consequences, no damage to anyone's reputation (except for some votes), and no one was even the butt of a joke here - it was the idea of the smart engineer with the engineered solution to a human matter which doesn't need solving. That's what we're laughing about.
Regarding all the talk about ridicule, it was intended as a rebuttal to the idea that humor at someone's expense is _never_ appropriate. It is appropriate and even healthy for a society to be composed of individuals who can withstand a bit of a ribbing when they unwittingly find themselves representing human folly in one of its many forms, and who can stop and ask, why did that happen?, and potentially learn from it, if it came from a moment of lacking self-awareness, or if it was just bad luck (everyone - everyone - winds up in the crosshairs sometimes.)
Truly no ill will meant, just wanted to defend the social function of ridicule. It probably ended up being a lot more of a well, actually thing than I intended. Which I do get teased for sometimes.
I think the issue here is that I fundamentally disagree with the direction of this particular ribbing. I think we should be sensitive to this kind of naive questioning, which comes from a good place (wanting to make teaching easier and more effective) and can lead down productive paths where our priors are deconstructed such that we change what we think of as possible. We work in an industry where rabbits have been pulled out of hats and geese have laid golden eggs, by which I mean people have put their minds to hard problems and have come up with solutions that before seemed impossible and would have faced the ridicule that you're defending. To the extent that ridicule causes people to be less curious, we should be very careful about deploying it.
I'm certainly no fan of shutting people down for asking questions. I think we may be interpreting the context of the question differently though. The parent poster was recounting having worked as a math teacher, stressed out every day from the planning and grading demands of the job, working in excess of 12 hours per day, earning a salary low enough that colleagues without a spouse's income to help out had to work a second job - and who still has nightmares about being back in the classroom. And then moving on to a software job with double the salary and half the stress. I read it as saying teachers should be appreciated / paid more and the response as why are teachers still doing unnecessary grunt work, making lesson plans and grading papers? Isn't that dumb work that a software engineer could just solve for them?
(Also, following through on this, if such a solution - computers planning your lessons for you, so all you have to do is what the smart computer tells you to - were to become reality, the lack of teacher appreciation / salary, which was the main point of the parent comment, would not improve at all - it would likely get worse, with teachers getting paid less for fewer available jobs. The deskilling of labor would probably follow, reducing teacher roles to individual, specialized, repetitive tasks, since it saves an enormous amount of money if low-skilled workers can simply be swapped out if they don't work out.)
FWIW, as the person who raised the criticism of the snark, I also am a former math teacher. I have experienced just how much more valued I am as a software engineer. I have experienced how vastly better my work-life balance is. I, too, have observed how folks in the tech startup scene can approach complex social problems with overconfidence in tech solutions. So I have a similar claim as the OP to feeling insulted and entitled to snark back. I just don't think it's the right thing to do, from the standpoints of how I aspire to treat people and from a pragmatic standpoint of seeing the changes I want.
As you admit, the question asker might very well have been genuinely curious. The answer to their question doesn't seem blindingly obvious to me (although I can certainly speculate about good reasons that grading isn't automated). I can't tell what you mean by "unacceptable" (perhaps there is some cultural taboo you're referring to here?). And it also doesn't seem "dangerously naive", which might apply where someone asking this question had decision making power in a critical situation, but clearly doesn't apply here.
By the very criteria you lay out, this doesn't seem like an instance in which ridicule was necessary or good. To me, it feels like a weak justification for bullying someone who thinks differently about something you obviously feel very strongly about.