Could be, but had this been the case, in the past?
I always had the impression that the smaller carriers got OS updates last.
The running joke was that AT&T always got any update last. People on Verizon always seemed to feel they got updates last. And for years, that kind of was the case.
But in recent years, that has changed. Starting with the S8, Samsung has been generally more consistent in getting security patches out, and in the US, they went out to the carriers first.
While in the past, Samsung would update unlocked phones first, they began holding off updating the unlocked models until after all the carriers updated.
The question of large verses smaller carriers may just be a matter of how much a carrier modifies their version.
The OS itself from Samsung is universal for all devices. The accompanying CSC file is where carriers add or lock out certain aspects of the OS. AT&T has a lot of tweaks. Most of the smaller carriers likely have very little, if anything, they mess with, and probably just use the default CSC. That could be why they are quicker to release the update once it's available to them.