I think it's just an awkward approach on the author's part in trying to show how strong their regular state is nowadays. It's been quite a while since they've done anything of note in base and he wants to show their progress. He just does it in the most awkward way possible.
From what I can see, the boys attempting to fight 18 in base and not doing so badly is the best indicator of where the base powers are at. Whether 18 is stronger or the boys are, they should be in some general vicinity of each other.
Also, Vegeta declaring he would win the tournament in base with 18 standing literally mere feet away from him seems to suggest he'd probably win.
Piccolo is a whole different story. Piccolo held his own against a Cell Jr while a fatigued MSSJ could not. Even if the author is attempting to retcon, I am not able to accept it.