The reason for that statement is that the main thing to look for in Black Market kingdoms is not the contents of the Black Market deck, but the cycling available in the kingdom. If you can cycle a lot, you can play the Black Market and the things you get out of it enough that it's good no matter what. And if you can't cycle fast enough, even if you get something good out, you won't be able to play it enough to make a big enough difference.
This is mostly true, except that without good cycling, there is some probability that you will get something good soon enough, depending on the contents, and you have to weigh this against the alternatives, so it's possible that you might want the Black Market even when you can't cycle that well.