Most of us don't realize that we only have major and minor keys. That's it. Dorian is not a key. It's a mode.
I ended up more confused than enlightened.
Although he dismisses it out of hand, Dorian
is as much a key as major (Ionian mode) and minor (Aeolian mode).
What's odd is that he treats Ab and G# as different keys. Notation doesn't change the sound of the note. But if you're including enharmonic equivalents, why arbitrarily stop at single flats? Bbb a valid key as well under that definition, which means there are an
infinite number of keys.
I'd suggest that if you argued that modes aren't keys, there are only 12 keys - "key" refers to the "key note" of a scale, and there are only 12 distinct notes on my piano (when it's in tune).