What you need to understand is that "lemmy instances slrpnk, lemmy.fmhy.ml, beehaw.org collectively are reddit" is not correct. The proper analogy is that beehaw.org alone is reddit. And then beehaw.org is linking up with other "reddits".

The technology communities in those different instances are their own thing. They aren't "the same one community split fragmented" they're separate communities.

so while I can post in here on technology@beehaw.org it's very much the case and obvious to me that it's separate from the magazines we have here on kbin. we have our technology@kbin.social which is our technology community. and this technology on beehaw simply happens to be another technology community that I can see and participate in.

In practice, what results is that people interested in these topics will generally subscribe to all of them if they want to see all of the content. but they aren't the same thing.

I know y'all here on beehaw have some pretty emphasized posting guidelines that simply don't exist elsewhere on the fediverse. as a result, whenever I'm in a beehaw community I make sure to not kick the hornets nest (sorry I couldn't help but make the pun). but on the communities here on kbin? yes I happily participate more comfortably.

tl;dr: they're different communities, not the same community split among instances.

edit: it's also worth noting that us kbinauts aren't even using lemmy, and neither are the mastodon users who sometimes participate in these threads.