They have a similar integration with Bitwarden that I've used a bit. I ended up stopping though because I rely on a catch-all and just give out companyname@ or something generic like work@ or family@. Sure it's easy to guess but I haven't had any spam issues in the ~15 years I've been operating this way.
Nobody actually gets my Fastmail login address though. I picked a random string on one of their domains that's literally only used to sign in. A fun little added obscurity feature.
How so? The three biggest things I attribute to Google are search, ads, and their mail/calendar/drive/docs suite. The only thing I see Proton doing is the last, which serves as an alternative to more than just Google.
(I ask this as someone that does not use Proton as primary for anything)