I don't remember Broly ever getting healed then, so I don't think he had gotten that power-up. Also, I don't think that Adult Gohan is way weaker than Teen Gohan. Weaker, yes. But I wouldn't say way weaker, especially since Teen Gohan would have gotten a boost from getting hurt by Cell. So I don't think that Adult Gohan is much weaker than Teen Gohan at that point.
Also, since Broly wasn't healed, I don't think he would be at full power, so that could have something to do with it.
As for the Kamehameha, it wasn't charged up very long, so that could have something to do with it too.