More careful reading would've revealed that nobody said "there isn't enough power on USB to run that light bulb", rather "do that" would obviously be the end result reported about.

Further, if you ran a 2.8V bulb off 5V it'll use more than 0.3a and immediately burn out so obviously it was further current limited. Given a concentrated enough heat it could pop a little but not enough to make it worth the effort or have a snack in a reasonable amount of time.

The disappointing part is to generate more heat to do it instead of using the waste heat from the proceessor. Mount a little aluminum pot with copper bottom over the processoor and you'd have a more suitable and more green popcorn maker, if only you found the load level that keeps the CPU from frying itself.