Maybe (I'm guessing here, no idea if it's the truth) he's been holding back and is way stronger that what their party has seen, but will have to unleash his full power against Dantalion, which somehow Noel knows about, and which will cause his party to feel both insulted and betrayed.
I was thinking something similar... It feels like Leon intentionally hesitated to turn their party into a clan after they got their dragonslayer street cred, both to keep his full abilities on the DL while also giving his friends time to catch up (which they potentially never did/could). If they created their clan that early on, the country would've compelled them to take on more dangerous Abyss requests, thus putting his comrades at much higher risk of dying than himself. I don't think he's the type to risk the lives of others over his own like that and, unfortunately for him, that also makes it look like he doesn't have faith in their abilities to keep up with his own. Once they find out how far behind they really are, just like Noel told them, the party could start to unravel.
It's ironic that if this is the case then Noel is the one being honest with them while Leon is lying to them by hiding it. What would you think if you were in Kaim's, Ophelia's, or Vlaclav's shoes? From a certain perspective, Leon is definitely looking down on them.
Looking forward to see where this goes...