Having passwords means storing them correctly and still implementing some form of reset -- and if email is a weak point, they would have to 2FA prompt you to reset your password, or not use magic links, or handle such issues out-of-band. They're probably trying to avoid auth/password support by outsourcing this function to Google for the time being.
I agree though, the user experience that is hardest-hit by this is mobile, where iOS now supports much better integration with password managers than was allowed previously...
But on the merits: not storing the password is safer than storing it so it's a valid claim.
And it is a 2fa mechanism to a tee (https://en.wikipedia.org/wiki/Multi-factor_authentication)
It’s unusable for me as well, but for a similar reason on the computer. When I’m on a computer, the last thing I want is to be forced to login to email for a code since I use web based email (which I don’t keep open all the time). Even if I were to use a desktop client, I wouldn’t have it kept open all the time.
These are “chores” added to the user experience.
Specifically, the login is a randomly-generated one-time code sent to your email address. Notion says this is more secure than them storing a username+password, but that's a dubious argument. They've also said this is two-factor auth (lolno). A side effect of this is that Notion is unusable on my mobile device since I have no email on it.
I really hope they implement a more traditional login system. Until then, I'm sticking with Evernote. :(