@Penjuugo That's the main theme of his character: he's not the strongest guy in the universe like Angie insists, but he's still a lot stronger than he gives himself credit for. He doesn't appraise himself very well due to how he lost his leg literally at the start of his adventuring career and he always has convenient excuses to downplay things when he accomplishes a feat he thinks is beyond him. Ex. He hides behind his career starting and ending in E-rank, but skirts around that he's not part of the guild's ranking system anymore and hasn't been for decades so he has no idea how they would appraise him over all those years nor where his ranking would be had he not quit so early on. That's an excuse Sasha immediately saw through, pointing out he's technically only E-rank because he never took any of the exams and applications for promotion to higher, not because the guild determined him unfit- she attributes WHY he never did entirely wrong but that's neither here nor there in this context.
But for all his excuses, we see unrelated people come to very different conclusions of their own about him: Sasha's a AA rank, is stated to be among the strongest adventurers in the Bordeaux region and promotion to S herself in all but inevitable, and legit hasn't bested him in a duel either. But Bel doesn't have the familiarity excuse for her that he does with Angie; and Sasha's now also part of his fanclub and appraises him quite high based on her own experience with him rather than just Angie's tall tales- and we don't have reason to think any of the Bordeaux sisters are poor judges of character and talent, considering one of them's entire schtick is farming and accumulating highly skilled people.
Even in this chapter's duel, even if Angie was phoning it in to 'prove' her point to Ane and Milly, it still ended up looking like the same level of ability they see her display all the time to defeat entire hordes of bandits/monsters, slay dragons, etc- that is to say, even if Bel is not as strong as Angie wants to think he is, he's still a lot stronger than most out there if he's still able to actually respond to her moves after what is effectively a 10 year gap in growth since they last dueled. He's well past the prime of youth and is for all intents and purposes 5 years weaker (albeit his regular training is mitigating some of that but we see he IS slowing down with his age), while she's got 5 more years of growth under her belt. He dismisses his victory as Angie not being serious and having failed to correct bad habits he knows about, but doesn't acknowledge that familiarity with her doesn't mean much if he physically can't keep up with her, which he can still to some extent do even at 42.