The video that I link says that he became weaker after he absorb the kai's that is what I am basing my kid buu is stronger than super buu.
I dont think he was scared to fight super buu, I think he was just trying to calm Vegeta down to have him think rationally and convince him that they need to fuse again to win. The super buu without the absorption of gohan and gotenks would not have been able to beat Goku. I think he was basing the power level of super buu with the absorption of the other characters when he said he was too strong. When they freed everyone he thought buu would be weaker but he became stronger.
He may not be able to sustain being a ssj3 that long but he can hold it long enough to beat ultimate gohan. How long does it take to win if you really want to kill someone. In dragon ball, they sit and talk for like an hour and not power up all the way til the end of the fight instead of just killing someone instantly. He can hold being a ssj3 over an half an hour which is plenty of time to beat a lower class ultimate gohan. We also dont know how power goku truly is as a ssj2, that form may be even stronger than ultimate gohan. Goku been training in outerworld becoming stronger and picking up new technique while gohan was being family man. You saw how when they were in grand kai planet he just walk over and picked up the sword like it was nothing.