I have never spoken to a single fighter or trainer in my nearly 23 years of training who did not believe in ring rust because it's a very real thing. There are entire training regimens based around eliminating ring rust. Trainers and fighters give seminars on how to mitigate it.
Perhaps fighters are just trying to stay confident. But each time I see a fighter interviewed and asked about it, they either have said it isn't real or it is hardly a factor.