"Super Kaio-ken" doesn't have an official set multiplier, so it could be a minuscule boost; that's entirely speculation. The scene isn't open to interpretation because "it's filler," isn't an excuse. Taking the whole plot hole aside if Pikon were to be stronger, which he isn't, then look at the actual evidence in the fight. Do you even see Pikon and Cell fight? No, you see him being taken by surprise. There's nothing subjective or open to interpretation about how Cell's beaten. This is the same as Majin Vegeta cold-clocking SSJ2 Goku to go and fight Buu. Does this make Vegeta stronger? No, they're very much equal in terms of power in their SSJ2 forms.
There's no plot hole about Goku being at SSJ2 strength, because Pikon wasn't stronger than Cell, simple. Goku charges Cell in his... In between state...? Regardless of how we treat his form, there's no indication he'd even powered up to his maximum from the Cell Games. If he had, Pikon wouldn't have been surprised in their fight. Plus, look at Cell. He was likely at the same level as Goku so he could have a little fun, instead of a one-hit overkill. Note there's no sparks around Cell, or even a power up. Note that Pikon flies out of nowhere from behind Goku, running directly into Cell; take into consideration Cell's surprised face as he sees it a second before he gets kicked.
It's a very similar moment to Goku and Piccolo Daimao's fight. Daimao wasn't expecting Goku to come at him, Cell wasn't expecting anyone but Goku to come at him. Not to mention that Cell wasn't even charging at Pikon, it's not like Cell was being cocky and somehow knew Pikon was going to intercept him and changed his course of trajectory towards him. Just look at the episode, it speaks for itself, there's nothing to interpret other than Cell was taken by surprise.
Cell states Goku's strength is nothing, also indicating the reason as to why there's no power up from him.

Cell charges at Goku (No power up from either side, just SSJ Goku, well "in between SSJ" since the animators changed their minds".

Goku is genuinely surprised by Pikon interfering.

Cell is discombobulated by what the hell is coming towards him.
