You're I stand corrected. Thank you.
This is very interesting, and since Android Architectural changes I can see how this is possible.if you can't unlock a bootloader you can't flash a GSI. also with Qualcomm Samsung phones recently, even unlocked phones bought directly can't have their bootloaders unlocked the normal, easy way. it's not just carrier phones that can't be unlocked it's all newer Qualcomm Samsung phones in the US.
I'll look into mentoned Qualcomm Samsung, Very interesting and potentially problematic vendor decition.