@de0u and @tnk479 did you try ''Method 1'' in my post earlier?
First thing I try is to go to Google's Stock ROM installer and plug the Pixel into a laptop using a Chrome based browser (Chromium, Brave work).
Check for an update but don't install anything. Just check if you see anything relating to ''Verizon or T-Mobile''. This indicates that it is originally from one of these carriers and OEM unlock will not be available no matter what you try.
I have had experience with phones from both Google directly and 3rd party sellers who say 'Factory Unlock' and yet when first running a check with the installer from Google, I see the dreaded Verizon/T-Mobile update.
If you see this, the phone is no good for unlocking the bootloader no matter what the seller says.