Hyperbole and lack of research.
It was stated by the omniscient narrator, and seeing as it's not disputed by any higher forms of canon, it goes. Just because you don't like it, doesn't mean that you can just shrug it off as hyperbole, that doesn't quite cut it.
It was BS.
Willing to provide an argument as to why?
It said a Sith student was using vaapad at one point in the novel.
A mistake on Drew's part, doesn't make any of the rest of the book less genuine or anything.
Maul would not get tired
Yay for unsupported assumptions. I've heard you parade around how Maul is beyond fatigue, care to provide a argument, because nobody is beyond fatigue, everyone eventually would run out of energy given the right amount of exertion.
where Kas'Im was stated to be a victim of fatigue.
Where was that said? That is just ridiculous, considering that Kas'im kept his body in better condition than any other sith lord around at the time.
Maul knows how the double-bladed lightsaber works and has been using it since childhood.
Within the first few passes Bane knew he couldn't win. Kas'Im had trained his entire life for this moment. After years of study, he'd mastered all seven forms of the lightsaber. Then he'd honed his skill for decades, perfecting every move and sequence until he had become the perfect weapon and the greatest living swordsman in the galaxy. Maybe the greatest swordsman ever. bane was no match for him.
What seems more impressive to you?
Maul can resist lightning with or without a lightsaber,
Erm, feat wars anyone? How is that relevant to a saber duel.
has obliterated the black sun with ease (which includes many of the "Galaxies deadliest" people)
Quantify and substantiate this please. How did he do it, how many people did he kill? Sure, from what I remember (I read the comic quite a while ago) it was somewhat impressive, but at the end of the day it doesn't really speak for his duelling skill. You're gonna have to do much better than that.
is a master of teri kasi
This is great and all, but still nothing on Kas'Im. I'd pick a master of all seven forms, as well as a master of the dual sabers as well as the saber staff over a master of Juyo and Teras Kasi.
and is from a more powerful era where the dark side isn't spread thin and weak.
This is a pretty ridiculous way to try and downplay how powerful Kas'Im's era was; if anything 'the darkside being spread too thin' had more to do with the sith philosophy and how strong and stable the order was, it has hardly anything to do with the personal power of anyone in the order.
Also, this is a discussion on saber skill, not force power, so your point is moot anyways.