That may just be the sort of thing they didn’t or don’t really anticipate people to be doing, and it’s a stretch to say they’re advocating for it. You’d think, even, that after a few attempts waking up late at night for a groggy review session one would start to get the impression that it’s maybe not the best for memorization (or be put off doing it regardless just because it’s unpleasant). Setting that aside, though, in general, the WK team do seem to be pretty good about reminding people to maintain a manageable, sustainable pace rather than trying to race through the levels as fast as possible.
Although, the guide written up by @jprspereira, the user quoted in the article that @chellykins shared earlier in the thread as having completed WK in only 368 days, also shows how, with WaniKani’s SRS intervals being what they are, it’s entirely possible to plan a routine that both allows one to go at top speed and still only do reviews during normal daylight hours without sacrificing sleep. To quote that guide: