I just did it from Firefox on Linux in a private tab near Washington, D.C.. Fake name, no phone, no backup email. I was able to log out, sign back in, and send an email without any trouble.

No doubt they're letting me through because some security heuristic says I'm a real human, and I'm sure they'd eventually make me provide a number if I continued using the account (this happened to me with my university G Suite account a couple years ago and I needed to contact my IT department to manually disable the phone challenge), but so far I can't see any evidence that they're doing anything unreasonable.

Perhaps they're requiring you to use a number because you've tested it a lot.

I thought the same but I just tried on firefox desktop (Windows) and spun up a new google account with email, password, fake first+last name and fake bday. Really, I was expecting to be stopped at "Phone Number required" but it is indeed optional.