It also says his strength hadn't changed since he was a kid:
Many years after the battle with Cell, Gohan had become a high school student. Due to either Chi Chi's influence or him neglecting to train, his strength hadn't changed since he was a boy.
So his power hadn't changed since he was a kid, but he also doesn't have the same power as he had when he defeated Cell.
According to Vegeta, Gohan looked pretty rusty when he showed up at Capsule Corp. That seems to just be an assessment based on appearance. Gohan becomes Super Saiyan, which doesn't seem to raise any alarms. Super Saiyan 2 is the only thing that stood out in terms of showing Gohan's decline in power--and even then, only Vegeta noticed it. Lol this part is stupid.
Now if you believe Gohan is a Super Saiyan against Dabra, then Vegeta confirms that he's inferior to Super Saiyan Kid Gohan. What I don't understand is why would it have been obvious when Gohan became a Super Saiyan 2, yet his pissed off Super Saiyan wouldn't have been a clear hint he was much weaker? Vegeta got mad during the fight with Dabra as if it was unexpected when it should've already been obvious. Or you could assume he was raging on purpose to get Bobbidi's attention. Either way, this part is dumb.