Statistically, there's nothing wrong with the null hypothesis being p=0.9. It's probably not a test you would see very often in practice because usually there is nothing interesting about p=0.9. But if you wanted to test whether or not p=0.9 for some reason - any reason, setting the null hypothesis as p=0.9 is a perfectly valid (frequentist) way of doing it.

Hm, good point. Since the usual thing is .5, the claim should be the alternative. I was thinking in terms of trying to reject their claim (which it wouldn't take much data to do), but I do think my setup was non-standard. I'll fix it later today