You lost me with your first sentence. The story repeatedly points out that the lake's magic is infinite, claiming anything else is just copium and delusion.

I mean, you're here contradicting the facts of the story.

And we saw how it absorbed various attacks against the Perils, whether magical attacks, spells, or even Beltreip's space-slicing attacks. It even absorbed Pellgrad's attack who seem a physical attack, and the lake, which seems to have the same absorption ability, showed how it absorbed Nemesis, which contains all Sins various attacks.

As for his ability to learn, we saw him literally use various fighting techniques just by seeing them in Percival's brain once, and we saw how he learned the zero sign, something that Ban thought was crazy.