I have an iPad pro (bought from apple so both internal e-sim and external sim slots are unlocked) and I use ATT. I was never brave enough try the internal sim. I always used the external sim with ATT. I have used the internal on other carriers and it works fine.
I don't know this for 100% but as I have read and thought about this, I assume that the internal e-sim that apple uses, along with their external apple sim has some sort of virtual ICCID that is not specifically assigned to a carrier. I am guessing it is a universal number that can be re-assigned to any carrier, as long as they release it. I believe therein lies the issue. Once ATT gets a hold they won't release it, so it is essentially locked to them.
The OP is correct I believe. iPad Pros bought from ATT have the internal sim locked to ATT. It is not the same as a sim lock that only allows a certain carrier, ATT does something or maybe just won't release it, but the end result is the internal cannot be reused. I am told it holds even if hard reset.
In the case of the one on the iPad Pro not bought from ATT, you can have more than one carrier associated with the internal sim and swap back and forth. I have a prepaid Tmobile plan and also added a sprint $5 day pass to prove that to myself. Once ATT locks the internal sim, it is limited to one carrier (ATT) and nothing else. Not saying that the AW3 will allow multiple carriers like the iPad e-sim (I am sure it won't) but I am saying that I don't know if ATT will lock me once I activate? The thing is I don't have a way to test. My phone is ATT, so until I sell, give or loan it to someone else I will not know if it is locked.
Once friday comes, someone please activate it on ATT, reset it and activate it on another carrier and then let us know how it works. If you could do that by early evening, so I can activate mine, I would appreciate it! 🙂 Seriously till someone does that, we won't know. I doubt that I will wait once I get my sweaty hands on my SS AW3 Friday.