I agree and disagree with those who believe ring rust is a myth.
On one hand, Rust is clearly real amongst most everything. Speaking for myself as a video gamer, if I even take a few months off from playing games, it usually takes upwards to several minutes or longer before I can really get my groove and timing back. I assume this probably applies even more to a highly competitive and physical sport like fighting.
On the other hand, the one big difference with combat is that fighters train and spar like madmen for months before competing again. So in a sense maybe guys like Dom and Woodley are right in that they believe that any rust you had should already be wore off by the time you step back into the octagon itself.