Even setting aside Sign In With Apple, why are you consuming mental space and energy with a reused password and whether they should be throwaways or not in 2020, when all mobile OSes and desktop browsers integrate seamlessly with password managers?
Over here, I just use Face ID and 1Password does the tedious filing-and-remembering part that computers are good at.
I use 1Password on both my phone and desktop for logging in. Saving a new password on the desktop is easy. For the phone it seems rather tedious. Especially when, even on the desktop, many sign up pages have me submit 2 or 3 times because the username is taken, password too complex, or I didn't give them my phone number. I find it way less mental space and energy to reuse a weak password.
I know they're very popular, but there's so much long-term unintended baggage with using these singe signon systems...even assuming Apple's doesn't have the nefarious parts most others have.