Why are people saying that Lemmy is free of corporate interest?
Can’t a corporation just enter the space whenever they want to? Can’t they start or even buy out larger instances? Even if Lemmy does take off, wouldn’t this inevitably happen anyway if the space gets popular enough?
Technically it's not since any corporate entity could set up an instance and join the Fediverse, there's nothing to stop them. However they could get blacklisted by other instances for whatever reason. For example if Meta were to set up a giant server and plop it on the fediverse, all the admins could collectively say screw those guys and defederate them across the whole thing.
So in that sense there's no one corporation that could take control. The community is and always will be collectively in control. The philosophy of the Fediverse is FOSS so if a corporate entity tried to monetize an instance, other admins would be pretty quick to block it.
OK, but what if Meta's instance, due to their vast marketing power, becomes an order of magnitude larger than the rest of the fediverse (I don't think that's an unreasonable fear); some instances start federating with it justifying that it brings you both worlds, it becomes an increasingly hard-sell to join those instances not federating with it and they became very niche or die out. Consider how Android started out as a nice neutral FOSS project and, while still technically FOSS, once Google became dominant it became Google spyware.
Many open source projects have died that way, embrace, extend, extinguish. True it's hard to say for sure that Lemmy and the Fediverse will never fall victim to that. However there's lots of open source projects that have endured without corporate corruption.
To be fair, AOSP is open-source and free of Google's services, but said services deliver ecosystem integration - and now even core functionality, as they're deprecating the stock dialer app.
You say "to be fair" but with full respect I don't think there's a fairer way to put it than how I described it as "technically FOSS". The source is free but does it work to provide freedom to users?