Yeah and it also only works on your phone (or if you know how to make Google think you are on your phone) and in certain countries. All to my knowledge and based on my tests.
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.