Yes, that's what I mean. In fact the SE amp's power transformer is not nearly 100C on the outside, but if it were that would be OK. The transformer's internal parts are rated for 155C.
Almost all OTL amps are operated in nearly Class B, so the power dissipated in normal operation is much less than at full power into a resistive load. Therefor they do tend to run cool when run with music - they can get quite hot on the test bench. SET amps are necessarily operated in Class A and will run hotter. Even transistor amps, if they are Class A, will dissipate a lot of heat.
In addition, many designers like to operate the components cool. This is in general good for the longevity of components, but it is far from the only important design parameter. For power transformers in particular, you can run a transformer cool by using fewer turns of thicker wire, resulting in low-resistance windings. But the magnetic field in the core will then be very high, resulting in transformer vibration, nonlinearities that contribute to "buzz", and large magnetic fields that will induce hum into the nearby circuits, I believe it is better to give the transformer some magnetic headroom as well as some temperature leeway, rather than concentrating on a single parameter. Engineering is all about compromises!
