I think that in practice, every operator of a messaging app will have their own node. There will be Signal, WhatsApp, Messenger, etc. nodes and some small node

I think that in practice, every operator of a messaging app will have their own node. There will be Signal, WhatsApp, Messenger, etc. nodes and some small nodes for businesses, bots and similar. Yes, theoretically, if you DDoSed Signal, then Bob wouldn't be able to get the coins.