Thanks to everyone who replied on this thread -- very informative and on-point answers which make me glad I asked.
I do see the point of view that this measurement misapplies the meter because a signal with this high of a crest factor exceeds an item in the meter's specification.
However, I'd argue that one of the main purposes for the AC RMS measurement features is in mains power measurement, where control of power by way of SCR-switching is commonplace, not to mention other MOSFET-switched methods, all of which result in chopped sine waves. And where power is adjustable is just the sort of place you'd want to measure it, and its RMS value.
It would seem fairly inevitable that a user would be drawn into using the meter in exactly the circumstance where the chopped sine wave exhibits a high crest factor, without even realizing it, and get a quite bogus result. I agree with @Kleinstein that we would expect the meter to alert us to the high crest factor, and if set to autorange, just switch to a higher range.
I'm not sure whether it's feasible for Siglent to address this in firmware, but if possible, I hope they do. In my observation, it's the only meter, out of four that I had on this task, that had this behavior. (I suppose the others might have also hit a problem, just not at the exact same values. )