Maybe I’m missing something, but why would you trust any app to securely store your passwords (or store them at all) but specifically mistrust their browser extension?
2. Web apps are wild and doing wild things. Even humans might find it hard to fill the form correctly. Extension can only observe DOM. Who knows which inputs it will fill with my passwords? I don't trust myself to write that kind of auto fill code which would work reliably. And if I don't trust myself, I definitely don't trust others to do so.
I don't claim that extensions are malicious. That would not make sense indeed. Just that extensions are things I don't trust to be implemented correctly, especially with convenient auto-pilot mode.