Their data cap process is still odd to me...
Back when ATT first took away True Unlimited, I read their "fine print" & it said something like: It depends on how jammed up/clogged up our "airwaves" are... -- which gave no definite answer, which is probably why you got by sometimes with more, sometimes with less.
Early on I got 1-2 of those text msgs like you got but none since, & that's been several years, yet they continue to "throttle at will" even w/o a "warning text msg."