Nope, t-mo use its own database to check IMEI number belongs to wearable device. Unfortunately, that means only T-mo branded watches are in the database. This restriction was only put in place in early 2015. Before that, there was no such restriction. I guess a lot ppl use it for regular smartphone.
If you ever put you wearable sim in a phone, you will receive a message and the sim will be permanently locked and useless. AT&T has nothing to do with it. When Samsung Gear S2 first released, a lot ppl have trouble even with T-mo branded watch because T-mo forgot to put them into database.
Just ask @xendula. He was on T-mo wearable plan with his AT&T version of Gear S. Last Nov, he took the SIM out and put inside the Urbane 2, his wearable SIM no longer works with either device.