You're seriously missing the point, and actually arguing Lügner's dumbass excuses he tried to make to comfort himself post-Hubris loss to Fern.

Fern trained for years under arguably the most accomplished mage in all of history, who herself trained under the most famous mage in all of history. Frieren has doubled down on this and forced Fern to do most of the fighting the last several years in order to have Fern rapidly catch up in experience. Fern is debatably one of the best mages around, and certainly for her age, due to Frieren's honestly rather brutal training methods that are disguised as Frieren being lazy.

Fern has a strange attitude due to the harsh life she'd had until their journey started. She's just a cold and closed off person. But she's also a hard worker, and they've always shown that. She practiced zoltraak every single day for years and years until she could pierce that rock.

Demons train and practice and refine their specific spell of favor for centuries at a time and take enormous pride in it. They lament that "geniuses" exist like Frieren and retrospectively (lol) Fern. They are too stupidly prideful to admit that humans or elves could ever be better than they are at magic. They are literally fabricating reasons for their "unfair" loss. Fern is simply better at magic than Lügner. Just like Stark is a better fighter than Lanie. And Frieren has always been stronger than Aura.

Fern and Frieren (and Flamme) both trick demons by suppressing their magic. That gives them an edge and encourages the demons to fail, but they are by no means weak. Demons stagnate. We were led to believe that Frieren stagnated too, but instead she was following Flamme's order to build a truly monstrous mana pool.

Bottom line, the demons lie to themselves when they lose. They refuse to admit that someone better than them can possibly exist. They've fallen behind the curve of development in magic and they're very bothered by it.