If you only have the phone app then yes they are instantly downloaded and removed from their servers.
But if you have Desktop client(s) registered, then they need to hold onto those messages until you open your client(s).
That is why they have a 30 day login limit on Desktop clients. If they didn't they'd potentially have to hold onto messages forever.
https://github.com/signalapp/Signal-Desktop/issues/4730 https://community.signalusers.org/t/dont-unlink-devices-afte...