That JavaScript isn't a serious programming language is a decade old understanding of the ecosystem and a disregard to the current day use of the language. A lot has happened in the last decade to make JavaScript "more serious" like NodeJS, TypeScript, and React. Even on the back-end, it's becoming more and more popular.
Eh, I think they were talking about the UX for people that have JavaScript disabled, not about the JavaScript language.
To me it sounded more like "I can browse GitHub without JavaScript just fine, but GitLab doesn't even display the README, even though I'm using the exact same settings".
Sounds like a problem with the user, not a problem with the UX. There should be no expectation that a website be functional when JavaScript has been intentionally disabled. You never see people who have disable CSS complaining that a website looks terrible without any styling applied, why do people still expect that a website function to some arbitrary degree when they’re the ones who have deliberately disabled its functionality?
It's both a user and UX problem. It's like when some part of a website's functionality is only available through Flash.
You know some (nowadays "most") people will have it disabled, and you're indirectly saying that you don't care about those users at all, which is not a bad thing.
It's the users' fault for not enabling Flash, sure, because they're not your target audience. But whether you provide a good UX for your non-target audience or not, it's your fault. And again, you're not obligated to accommodate those users, but that doesn't change the fact the you could give them a better UX but chose not to.
So, from this you can know that the users GitLab cares about are only a subset of the users GitHub cares about.
