As a reddit user I can go to /r/technology and see all posts from any user to the technology subreddit. I can interact with any posts and communicate with anyone on that subreddit.
Sure, but what about r/AmazingTechnology, r/InsaneTechnology, r/AskTechnology, r/TechnologyProTips etc etc. You'd have to be subbed to all of those in order to see all technology posts. And you probably are, because there's no penalty in being subscribed to many subs.
In Lemmy, I understand that I can browse posts from other instances from Beehaw, for example I could check out /c/technology@slrpnk.net, /c/tech@lemmy.fmhy.ml, or many of the other technology communities from other instances, but I can't just open up /c/technology in Beehaw and have a single view across the technology community.
True. But in due time you'll end up in situation where few of these (or maybe even one) becomes the "go to" community, because it has best/largest discussions - just like on Reddit. We're still at the start of this journey. Also, the other instances are their "own thing". Maybe that's fragmentation, but essentially they might be aimed for completely different demographic (the users of that particular instance).
And all posts from all of these communities are shown in your home feed, so it's not like you miss discussions. There's no penalty for subscribing to all of them.
The only "fragmentation" that could happen is if one instance decides to defederate the other instances. That effectively "locks" their content from everyone else. And that is a shame. But it happens sometimes. Because instances are their own thing aimed for their own particular audiences.