The problem with that is that you can't do it by making straight chord slices of the the wafer - each cut has to terminate or it will go through the middle of the next hexagon. But it does sound plausible - surely lasers could do this more easily than retooling the whole lithography pipeline for rectangular wafers. But you'd think they would have thought of the idea.
You could do triangular chips with straight cuts. But I that would divide the area more finely, which is the opposite of what they need.
It varies, if you look at meteor lake [1] the different die go from 27..100 sq mm
(Compared to the h200 which is an insane 814 sqmm)
But I don't see any obvious advantage to small triangular die. Once the die are small if doesn't make much difference vs rectangular as to how many you can pack into a circle (for a given die area), and rectangle are much more convenient