I don't find that so objectionable, to log into their site you need to enter some credentials, _usually_ that checking is being offloaded from medium onto a third party you may trust more. In the case that you really need to access it where you don't wish to log into your primary email you will still need to login to something, their approach allows you to choose what sort of login security you'd like (maybe you want 2FA via google, maybe you want a more security lax service)

I think password managers are great but the service I see them providing that has the most benefit is to reduce the number of passwords the user needs to actively remember. The single sign on approach has it's weaknesses but the idea itself is pretty sound, you should legitimately trust facebook or google or whoever to know more about secure authentication than some random site... the fact that this service comes with a privacy leak and implies greater online presence tracking is terrible, but the core idea is sound. It'd just be nice to have a neutral party doing it.