The differences between a professional situation/drinking with friends/talking with grandmother are way too big compared to the differences between HN and Reddit. The latter two are actually extremely similar in how content gets posted and discussed.
In other words, there appears to be no reason why HN and Reddit folks should behave differently when on the opposite platform. These behavior differences are artificial and ad hoc (i.e. 'we want to keep HN free from obtuse memes.. because we said so!')
You don't accept that different online communities can have sufficiently different cultures that "memes" are more acceptable in one than the other, even if said communities have somewhat of an overlap in audiences?
I'll give you an unrelated example - I watch lots of developer conference presentations and read articles on developer blogs. My pet peeve is when writers include reaction GIFs in between some code example or one-sentence epiphany. I'm here to learn, not to waste bandwidth on some dumb five frame 80MB file. GIFs are for casual "throw-away" conversations, not learning resources.
I like that HN is a learning resource. I like that I can read perspectives from people in many different fields and across the various economic classes. I also like reddit - I like the memes, the in-group culture (when it's funny), the bots, the one-liners, what have you. But I don't want humor and generalizations to dominate HN comments; I want educational content to float to the top so that authors are rewarded for sharing their perspective. I can find funny takes on HN headlines on reddit already.
The point is that HN and Reddit are not extremely similar. This is more like a work place (more rules, more interesting), reddit is more like 8th grade recess (less rules, more fun)
In other words, there appears to be no reason why HN and Reddit folks should behave differently when on the opposite platform. These behavior differences are artificial and ad hoc (i.e. 'we want to keep HN free from obtuse memes.. because we said so!')