they dont buy any bitcoin theyre custodian does

And if the custodian says the bitcoin they received is real, but it is in fact BSV, microstrategy is OK with that?

They run nodes for validation but that doesn’t have any economic weight since they are not broadcasting transactions

Let me be clearer: they are definitely watching someone else hold their bag, but that doesn’t give them protocol power. Anyone can spin up a node to watch. Those nodes have no say over the mempool.

It does in fact give them power over the protocol. Their power comes from deciding which protocol to follow. If you want what they are selling (USD in this case) then you need to provide bitcoin that follows what *they* decide the protocol is. The set of market participants on that side of the trade have 100% of the hard power over the protocol.

The trade doesn't settle unless the node used for validation says the TX complies with the protocol. The receiver of the coins decides what protocol to follow, the sum total of everyone doing that determines what "bitcoin" is.