Good point. I still think there might be some implication about the relative quality of your results if and when you actually pass "the supreme test", but that is certainly not stated, and Ansel Adams might simply be saying, as others have, that landscape photography is very difficult, and hence explaining why there are not so many outstanding landscape photographs (relative to outstanding photographs of other subjects).
Yes, IMHO, while there are many photographic genres there are only photographers.
Also, despite the variety of photographic genres they are all complementary.
We may all tend to specialise in particular genres but often the best way to create a really striking result in a particular genre is to employ a technique, an approach, a philosophy that might be more conventionally thought of as belonging to another genre.
It is true that landscape photography can push the boundaries of one's skills, both technical and artistic, but it is also true, for example, that having good skills related to portrait photography can be very helpful, not because one can control lighting as is done with portraits, but rather, that one might recognise the circumstances of light prevailing and exploit it to the full.
For me personally I am finding that the more I experiment with genres other than the outdoor/landscape/wildlife stuff the more and varied ideas I have about how to do things. This is still a work in progress for me.
Tony Jay