Even if it *is* unlocked, I bet T-Mobile handles the updates.
This typically means that if you're not on T-Mobile (service), you can't get them, without reconnecting it to them.
Some carriers won't let a branded phone (from another carrier) operate for tethering, as well, that varies, depending on both the carrier and device.
I suppose they could be using the OP update, direct, but I sort of doubt it, the ones T-Mobile will sell will likely be a custom ROM.
These can be easy to miss, or overlook, potentially, IMHO.
I'm going to wait for an 855 powered phone, myself, it's really the next "big leap" forward, given that most of the manufacturers are sort of fumbling around, trying to figure out how to (mostly) make the same h/w seem unique.
I'm really, really hoping OP goes to a 4K display though, that's the biggest thing keeping them from matching the other flagships, I still use my 6P for testing, and it still has a better display (than my OP6), even though it's significantly older.