lol.. okay after posting I tried again for a while, this time spamming all buttons chaotically, and something happened! This allowed me to try some experiments and I've figured out what's happening (sort of): just pressing X doesn't do it. Instead, when I land I seem to have to press *backwards* (run direction) and X. This is the only way to trigger the super jump as far as I've found, simply holding down doesn't work.
Okay I'm an idiot: my problem was that I was holding the down key the whole time. I guess this should be obvious, but anyway if I let go of down when I hit X on landing, it works.. hooray! Sorry about that -- I'm going to leave this thread instead of deleting it in case it helps anyone else that's dumb like me ;)