If an attacker compromises the page or does a man in the middle and injects a form that isn’t displayed, it can trick your password manager to auto fill your login information and then send it anywhere. But that’s just one vector and if an attacker has compromised the server, there are a number of attack vectors they could take.
Just wanted to say I’m a big fan of SMUI. I admire and appreciate the work you have done with it.