You guys are really over-analyzing that line.
I agree it was weird, but the translation as mangastream has it makes zero sense as kkck has already explained. You have to remember this is the same group that had everyone melting down about Escanor storing power, because it was a complete retcon of how his powers were established to work, and it turned out to be wrong anyway.
The proper translation, that actually makes sense, says that he was shocked that she said something so stupidly obvious, which makes sense. The same way we except the translation about the accumulation of stress rather than power, because it actually makes sense.
The databook already says that Escanor has never met anyone he thought was stronger than him, so there is no super secret demon king level badass out there we don't know about or, even a Gawain in this story. Saying that he thinks there is somebody stronger than him that we don't know about directly contradicts the databook.
I know this is a battle manga and people like to theorize about stronger people all the time, but unless a retcon occurs Noon Escanor is the strongest Taizai character.
Yes, I think the GQ and DK should be stronger than him honestly with their feats, but he was given the title of strongest by the author himself officially with the chapter title, and the translation of his "One" form, which means number 1 in heaven and on earth. So if Nakaba says he's the strongest he's the strongest feelings be damned, or even if you want somebody else out there to be stronger than him.
So yeah. Unless something gets retconned Noon Escanor is the strongest being for one minute.