I think you could create a system that's resilient to such issues even with federation (not saying it's easy, though), and Matrix actually has a solution in the works for this – decentralised user accounts [1].
And all of this makes me wonder – maybe it's better to re-implement something like Mastodon on top of Matrix. If Matrix adopts decentralised user accounts, that would seemingly solve such issues automatically. There was a POC Matrix based Twitter clone demonstrating this, actually [2] (but without the decentralised accounts yet).