He actually didn't say this, and nothing in the anime or manga suggests this
Here's a few key points that prove this
-goku was on par with gotents ssj3 or slightly weaker
-goku was not stronger than super Buu, he would have lost or being a better fighter he may have won but not because of being stronger
-gohan was stronger than super Buu by quite a leap, he actually wasn't too far from buutents
-buuhan is the strongest Buu no contest, before kid Buu came to be he reverted back to super Buu losing all that power from Gohan, Goten, trunks and piccolo then further lost more having fat Buu pulled
-Goku destroyed the earrings and was happy to fight kid Buu, he was prepared to fuse with Hercule just to gain a scrap of power against buuhan
GT is not canon to the story, it's deemed a fun side story set in an alternative universe
GT fanboys seem to forget this and I'll lay it out
-the events in super never happen In GT so as they've grown immensely stronger in super this does not mean they've gotten stronger in GT they remain the same which end of GT Goku is not stronger than Goku from super not even remotely close
