I wonder if Purism, being a MVNO, has bargaining power with AT&T that the Average Joe does not have. For example, Purism tells AT&T they will be a MVNO on their platform (which I assume entails some fiscal benefit for AT&T, probably from Purism), but only if AT&T allows them to have certain modem models (e.g. the BM818) be allowed on the network. Thus, AT&T has a whitelist that they will allow a bunch of approved devices on, but they can manually make exceptions for IMEIs that Purism has on its AweSIM MVNO. The benefit for AT&T is presumably monetary compensation; the benefit for Purism is they can offer a wireless service in the USA that the Librem 5 actually works on.
I think I’m still going to try a cheap T-Mobile prepaid plan before trying AweSIM for $100/month, but I’m less and less hopeful for the Librem 5 will be usable by anyone other than tech nerds with a bunch of extra money laying around.