You're not ignorant! It actually is for short periods of time, it's just that after doing interval training for awhile your anaerobic threshold goes higher & higher (to a point, of course) which in turn increases the intensity you can workout at & still be in an aerobic state. Apparently you should do interval training once or twice a week only, so as to not pound your body into the ground...but once or twice is all you need to get the effects. It can also be considered cross training since you are working out in a different manner, even if you're doing the same type of workout, i.e. stepping, running, etc. Of course you should also change the type of workout at least once or twice a week too, but that's another subject.
Also, strength training is considered anaerobic because your muscles are eventually working without oxygen, rather than your lungs etc.
So many possibilities, so little time! LOL
Ruth;-)