Sure, it's possible there are secretly stronger beings, but he's still stronger than any being has ever even heard of on this planet.
You can't "barrier aside", that's a HUGE mitigating factor! The dragon didn't/couldn't fight back. And he got none of the dragon's abilities for eating him.
If he ran into a demon king, or another dragon that DIDN'T have all it's powers sealed, I see no reason why it couldn't instantly destroy him. Except for the one dragon that was willing, he hasn't been able to eat anything that he didn't kill first, and I don't think any of his current powers would let him defeat a monster that strong.