I have never really heard of this "rule" because with my training you were discuraged from pushing it. So, in other words there may have been something mentioned refering to this "rule" where you "could" cut the flash time in the higher temp, but there is really no reason TO cut it.
The epoxy/filler test we are discussing had something go wrong, don't know what, but something. Your example would mean that the fillers heat help "kick" the epoxies cure up a notch which could have an effect on adhesion in my opinion. So you may be on to something. I do think it was that and other issues where the "planets where aligned" to create the failure.
If we look at the test for what it is, it is still valuable. If you follow the test, and apply epoxy under the same conditions you are not getting the improved adhesion qualities of the epoxy. If you have to apply the epoxy in that way, you are better off applying the filler over the bare metal and not using the epoxy. It looks like he proved the very reason why 95% of body shops in America apply the filler over the bare metal, they don't have the time to apply the epoxy primer properly.
THAT it did prove, so it is a valuable test.