free people from proprietary gardens, yet FOSS has actually been one of the biggest creators of such gardens

Forgive the nitpick, but FOSS is not creating walled gardens, companies are. (After all, software has no willpower… yet)

Indeed. If a big instance like lemmy.ml was to be shut down all the communities would be lost. This is simply not sustainable. Why would users put effort building a community if it could be gone at any time?