I bought the Pixel 9 from US Mobile last year during that insane sale and unlocked the bootloader and installed GrapheneOS. I didn't keep a US Mobile sim in it for 60 days either. I activated it initially with a Dark Star sim then turned it off for a month. Saw a post on their subreddit that some folks got theirs randomly unlocked early, did a factory reset on mine and I could unlock the bootloader after connecting to wifi. So, it is possible but ymmv.