Same with me. I was chatting with Xfinity tech support and they said all the Pixels were compatible. But when I put in the IMEI, it says no. Verizon says is unlocked, and is running Android 10. Only thing I was thinking is, my phone was officially paid for this month, and Verizon says they unlock devices 60 days after purchase. So, I was thinking, I have to wait 60 days for it to REALLY be unlocked.
Ticks me off too. I could save almost $40 by switching, but they won't take my tablet either. So, if I got a new phone and a new tablet, I might as well stay with Verizon.
XM knows they have issues w/ Pixels and their IMEI checker, and they're trying to fix it. There are manual workarounds, but it has to be done by high level support. It CANNOT be done by phone or store employees. (Don't even bother calling.)
The big thread about it:
Comcast employee Chris in that thread can help you; send them a direct message. They got my Pixel 3a activated after a bunch of work.