The quote could also be from an omniscient perspective. I found a quote in a chapter focused on Bane that specifically says something he doesn't know. That proves that the book can go into an omniscient perspective at times and expand outside of Bane's knowledge. Plus, Karpyshyn said in one of his emails that whether or not that accolade was Bane's opinion or an omniscient perspective was debatable. So there is a good possibility that Kas'im's accolade was from an omniscient perspective.
However even if that accolade was Bane's opinion, this is the guy who spent hours upon hours reading up on the exaggerated accounts of ancient Sith, such as Tulak Hord, Exar Kun, and "many true lightsaber masters," and holding them at a standard beyond that of the Brotherhood of Darkness, which he despised. Kas'im's bladework impressing Bane enough for him to entertain the possibility that Kas'im could be the greatest duelist of all time is still really impressive.
Not to mention it's not just Bane's opinion that holds Kas'im at such high esteem, Kaan and Githany both agree that Kas'im is the best duelist in the order, Githany going on to think that Kas'im is their best fighter.
Not to mention that despite being plagued by only appearing in one source, Kas'im still demonstrates a high level of mastery. Kas'im defeated his master Nad'az before his prime, who was particularly very skilled at lightsaber combat foremost above his other abilities. Kas'im also mastered all seven forms of lightsaber combat and spent decades perfecting every move and sequence of these forms. Then we have the fact that Sirak himself was an impressive enough duelist to be capable of casually using sequences that blend several forms together and rapidly switch between forms, and when Bane stomped Sirak, he considered the notion that he could take on Kas'im in a serious fight to be preposterous, and this was before Bane even knew Kas'im's full capabilities with a saberstaff, much less with Jar'kai.
Not to mention Kas'im's speed is also very impressive, striking at ten times a second with a saberstaff, and striking even faster after switching to Jar'kai.