There are no rules, which was exactly the point of my comment. There's no studying of systems to be had, because there is no system. I said "I always tell people attempting to learn improv and piano to 'play what sounds good and don't play what doesn't."

The particular sentence you quoted sounds rigid on its own, but when put into the context of my entire comment, it's clear that I'm not trying to give a hard rule, but a general convention. Like I said... you should play what sounds good; and don't play what doesn't