There's been too many A-level athletes like Muhammad Ali, Michael Jordan, and others for whom time off clearly affected their performance on their first nights back to say it isn't real...
Maybe it's a bit overstated, and maybe dedicating yourself to enough strenuous amount of training in the gym can help shave off the rust by the time you get back to the real thing, but ring rust is definitely real. Like with anything though, it affects people differently and in different ways. So thinking you've disproven ring rust because "hurr so-and-so destroyed so-and-so after a three year layoff" makes you look like a simpleton.
I've never performed anywhere athletically, but as a huge video gamer, I can tell you gents that when you haven't played a specific game in a long time (or especially any game), it can take up to several minutes to get used to the controls again and get back into your groove. I can only assume that would be even more true for a sport that involves timing, speed, coordination, reflexes, etc. on a grander stage.