You are correct. The threading is the same. Commercial AR tubes do not "step down" ever so slightly before the threading starts on the tube. Mil-spec tubes do. A mil spec stock will not fit on a commercial tube, but a commercial stock will fit (loosely) on a mil-spec tube.I don't think the difference between buffer tubes is in the threading. i.e. any in spec AR receiver (and by inference, the XCR adapter) should be able to accept either a milspec or commercial tube. The difference is in the diameter of the rear portion of the tube, I believe. I think the milspec tube is slightly smaller. So, you want to match a milspec stock to a milspec tube/commerical stock w/ commercial tube, if possible. I think worst case scenario, you can put a commercial stock on a milspec tube (with some additional rattle), but I don't think you want to try to squeze a milspec stock onto a commercial tube.
So like I said, my understanding is that the RA adapter should be able to be used with either setup. Maybe Terra can come along and confirm.