Thermals generally happen when heat is built up over time. If the current bursts are too high, it can draw enough to instantly turn FETs into fuses; the ESC just doesn't have time to catch that quick of a rise in temps.
Yeah, a buggy on 6s geared to 70-80 may be ok, but I know you are going to be pushing the ESC hard. Not to mention the brake spikes at that speed/voltage!