Until today I thought they were interchangeable spellings.

I'm curious, is "woah" incorrect? If so, why?

Even as the instinctive prescriptivist that I am, I can't possibly get get worked up about permutations on onomatopoeia.

Not even something like "woe" for woah, or "hmf" for hmph? :P

You raise good points. :)

Actually I like "hmf" despite my usual distaste for the dilution of "ph" to "f". The "ph" in "hmph" seems incidental, so it does not trigger me, I guess!

There is a highway sign somewhere on I-95 for "Fenix Ave", which took me a while to realize was a vicious act committed by illiterates against "Phoenix Avenue".

Whoa is just older. https://www.dictionary.com/e/whoa-or-woah/

Dictionaries are descriptive of actual use, not prescriptive rulings. Both spellings are completely valid, the pedantry is incorrect.