I'm pretty sure the SDKSs are Apache licensed so not proprietary. Obviously the backend is proprietary, but that doesn't affect the SDK itself. You can pin it, fork it, vendor it, whatever you want, just like any other FOSS project that you don't control.

I think more the issue is that people started thinking of the AWS SDK as a generic open source library rather than what it should be thought of: an open source project run by a particular vendor who not only doesn't care about helping you use competitors, but actively wants to make that difficult. I would guess the truth is somewhere in the middle, but I think the healthy thing to do is treat it like the extreme end.