20XX is basically an over simplistic hyperbole. We don't even know if Fox/Falco are technically the best characters at the maximum of human potential, as we just assume that due to them being so amazing on paper. In practice their tech might not be enough to make them superior. It might not be humanly plausible for them to consistently out-perform characters that get more mileage from less tech (Jiggs, Marth, Sheik, etc).
Hell, I'd argue that we really don't know the capabilities of all the "low" tiers to make a call as to their viability. Pikachu, Yoshi, and Mewtwo have all had mains capable of doing amazing things, yet everyone chalks that up to the player and not the character. I don't think that the players of these characters are magical in any way, and their amazing performance is simply evidence that these characters are actually capable of performing well given someone good is playing them.
I'd also point out that a lot of these low tier mains have picked up other higher tiered characters, but despite the switch you don't see any significant tournament performance after the switch was made (with the exceptions of covering really poor match ups perhaps).