- Gohan said he wanted to become "even stronger". Therefore, this acknowledges his several month training past RoF, so he is stronger than he was back then.
- It was also emphasized that he was not lacking power.
- Piccolo acts a bit shocked for a moment when gohan goes super saiyan, but then goes back to normal instantly. Does this imply he is surprised by Gohan's Super Saiyan state?
- It is confirmed that Gohan was not or hasn't been as strong as Mystic since long time. Therefore, either Gohan's super saiyan states are nowhere near as good boost as they used to be, or Lavender is a joke compared to Basil and Bergamo.
- Super Saiyan 2 Gohan was losing to Piccolo.
- Gohan can't believe Piccolo's power level. So, this means Piccolo has far surpassed his past self
- Piccolo fights ultimate gohan for hours off panel
- Piccolo says that this power is not the limit for gohan, and they both training together will break the limit.
Other than that you may have point.
Most of these aren't simple plot points, since you're adding your own inferences to the mix. There are some relevant counterarguments, as well: first and foremost, Goku expressly says that Gohan's original power was higher than what it was at the tournament. By all means and accounts, Goku knows the difference between Super Saiyan forms and the Ultimate form; therefore there's a considerable chance that this Gohan was weaker all around (even as a Super Saiyan) and that this alone could've contributed to his overall poor performance against Piccolo even by limiting ourselves to this very episode. Another element that probably needs to be added is that Gohan was also still in the process of "being retrained" when the U7 Arc took place; at the very least, we know that he was therefore weaker than usual, at least back then. Then we also know Gohan has not fought Piccolo for some time and the he probably spent considerable time "doing research" and "being a scholar", so Gohan not progressing further also seems likely.
Other things are also particularly off: Gohan wanting to become "even stronger" doesn't also mean he was already stronger. Its meaning ranges from wanting to become stronger than what he is at present (alas, someone who improved over a weaker form) to stronger than ever. Piccolo also says when fighting Super Saiyan Gohan that Gohan can't “evoke (his) original strength", which would fit together with everything that's stated above. This could in turn mean Gohan wasn't putting his heart into the fight, that he was unable to draw his ki as a Super Saiyan, that he could not go Ultimate, that he was plain weaker or some combination of these assertions.
Piccolo being de facto stronger than SS2 Gohan (whom, again, looks and feels like he's still weaker than usual for a plethora of reasons) is also debatable. Gohan is also established to be someone who, apparently, isn't fighting Piccolo without being some sort of psychologically impaired fighter who is either:
A. Always distracted.
B. Suppressing his power for multiple reasons.
C. Someone who drops his guard very often (all four things are basically repeated a dozen times in total in the episode) because he tends to get cocky.
Then again, Gohan was
definitely edging out Piccolo in SS2 in the few moments they fought. However, after landing a hit or two he then got "cocky" by Piccolo's own admission – which, again, in the context of the episode was further clarified as Gohan suppressing his power, guard or both things - and the opponent was able to choke him (the "binding" part being something that Goku could do to Raditz back in the day). I wouldn't be surprised if the idea was that at least Super Saiyan 2 Gohan had quite the edge in power ("you're powerful, but you have a weakness"), but that his cocky attitude let Piccolo get the tactical advantage.
Lastly, this whole situation in part of the fandom looks and feels like "U7 Piccolo became stronger than Vegito" part 2. While it has been objectively established is that Piccolo is stronger than what Gohan remembered, thi is - perhaps predictably - getting extremely overemphasized (while downplaying everything else in the episode, as well) since no other means of comparison are established and we only know that Piccolo trained a little on his own. We know that he's most likely able to hurt the current Super Saiyan Gohan even without Gohan's mind playing shenanigans and that he could be able to hold his own against Super Saiyan 2 Gohan. Nevertheless, concluding that Piccolo had been improving his power tenfold in such a short timespan and off screen is absolutely premature, if not downright dubious.
Furthermore, if we want to get rid of most of the contradictions or headscratchers, "Gohan was still extremely weakened all along" looks much more probable on paper by piecing everything we know. While going somewhat against the feat of Gohan sparring with Goku, there's still no doubt that both Piccolo and Gohan were way more serious in at least some portions of their fight.
After rewatching the fight and paying due attention to the dialogue, I'm very, very inclined to believe that Piccolo is only appreciably stronger than an out of shape Super Saiyan Gohan and behind the relative SS2 Gohan, who's "powerful" but who has the critical weakness of not taking things seriously, the current Ultimate Gohan being far above him anyway. In short, Piccolo could be marginally stronger and the outcomes could have been the same without theorizing gigantic leaps in power.
To sum it up, I'm mostly certain that if Piccolo were to face Super Saiyan Goku, Super Saiyan Vegeta or any form of Buu he'd still play second fiddle (just like non-Ultimate Gohan would).
