I dont think that is correct.
You get 22GB a month before throttling. 10GB of that can be used for hotspot.
So if you use 10GB of hotspot the first week, you have 12GB of iPhone data for the rest of the month before throttling kicks in. The throttling is separate from each other, but the total 22GB is the hard limit before throttling may go into effect. I can use 15GB of iPhone, then I have 7GB left for hotspot before my iPhone starts getting throttled, but I would still have 3GB more of hotspot (full speed).
AT&T told me 22GB of high speed per line, 10GB of hotspot comes from that 22GB. Other than that, their throttles are separate.
I dont disagree with what you're saying: use 10gb of hotspot, you have 12gb left of overall 22gb for phone data before throttle,
BUT whats being contested/confusing of the 10gb hotspot, it was inferred by the employee that what you use of phone data, counts against that 10gb bucket of hotspot.
i.e if you use 9.5gb of phone data, you have 0.5gb of unthrottled hotspot before the throttle kicks in
I hope that isn't the case, but that was the way it was clarified when I asked related to that, out of skepticism of the too good to be true-ness.