My own experience, but also what I heard from others.

Here is e.g. a poll where the majority also voted for Google Login: https://twitter.com/Hi_Fabienne/status/1790393552268132742

So it seems to be based on their own experience. The parent comment says this:

"When I switched from user / password to OAuth I couldn't believe how many users choose it. I could deactivate traditional user & password login and nobody would notice it."

Based on the paragraph following the part you quote, it sounds like it's based on what they've seen in their own app.