Sort-of correct. Whilst it is true that motors have a maximum efficiency range in terms of rpms, it is the esc that heats up when you use less than full throttle, as it has to 'switch' at a higher rate, and that in turn generates more heat in the FETS which leads to thermals.
Least it isnt an old msc that would catch fire if you used less than full throttle 100% of the time... (those were the days
)...