There is no evidence. That's the whole point. There's two panels that show an injured Piccolo and an unscathed Cell Jr who aren't even in the middle of fighting, 0 statements about how Piccolo was performing and 0 reason to believe that Piccolo reached a similar level to Trunks and Vegeta.
A barely injured Piccolo. That’s both the point and evidence.
I’m not claiming they’re extremely similar in strength, just that the gap logically isn’t at all sizeable.
If you want more evidence, the fact that Piccolo was way stronger than Android Saga Super Saiyan Vegeta before he went into the Time Chamber should act as good support.
While the human Z-fighters all got knocked out and Piccolo didn't, all this means is that Piccolo performed better than said Z-fighters, which is obvious. The Cell Jrs also were told, and had ample time, to kill the Z-fighters but didn't, showing their unwillingness to "fold" Piccolo.
And yet they pretty much fail, in any measure, to take advantage of this supposedly large disparity against Piccolo of all people? No, this does not make sense. They try to annihilate people weaker than them, even if they don’t kill them, and get visibly angered at Trunks and Vegeta’s resistance.
For example, the first thing they do against Yamcha is
break his forearm. They would have done this level of damage to Piccolo if they were capable of it.
Again, they were knocked out, and with a few or even less blows. They weren’t toying with them after or anything because they wouldn’t even be conscious of that fact.
Saying it's clear that Piccolo should scale to Cell Jrs despite no evidence suggesting this is pure headcanon.
Saying he doesn’t even remotely scale is pure headcanon. This argument requires favouritism of certain traits the Cell Jrs have only partially shown.
They would, absolutely, “fold” Piccolo if he was so much weaker. Maybe not kill, but certainly crush him.
Not only that, but you’re talking about someone with regeneration here. If we go by Piccolo’s offspring, they have their parents’ knowledge to an extent, so why not rip apart the regenerator or at least do more damage to him, like Frieza (who Cell and, by extension, the Cell Jrs. are based off)?