glaive00 What they said was that they could temporarily lift the carrier lock, then I would be able to remove the OEM lock. Right now my phone says "Unavailable on carrier locked devices" in the developer settings -- OEM locking toggle. Is what they told me not accurate?
I am not a T-Mobile customer (and so far no T-Mobile customers are chiming in), so I am definitely not an expert on your situation. But the person you spoke with might not be an expert either. If you were talking to somebody at a call center, who may not understand the difference between SIM unlocking and OEM unlocking, but has a "temporary SIM unlock" button and does not have an "OEM unlock" button, and if pressing the "temporary SIM unlock" button resulted in you quickly hanging up happy, then maybe that was "mission accomplished" for that person.
I think it might be prudent for you to take the device, and all packaging and paperwork, to a T-Mobile store near you, and ask somebody there to OEM-unlock the phone. If that works, great! If not, maybe return the phone on the spot and get a genuinely-unlocked Pixel from the Google Store (or from Best Buy).