Mikasa said:
Are you sure?
Killua to Netero: Pitou is stronger than you
Netero: Pitou is stronger than I am
Netero vs Pitou, Netero casually swats Pitou away and never thinks of him again.
Gon: Hanzo might be older and stronger, but I am smaller, faster and more agile
*Hanzo 1-chop KOs Gon a split-second after*
Yes, he says it because he thinks there is a chance of him dying, doesn't mean it's inescapable.
The same reason Meruem saying "You had me in check mate from the start", didn't stop him from trying to escape. Only difference they're not trapped underground and this is not a nuke.
It's not shoddy lazy writing, it's shoddy lazy thinking on the audience's part. And he's taking advantage of that to build real suspense. He's simply proving again and again that a character saying something is not "word of god", which IS shoddy lazy writing done by most shonen.
People think that if a character honestly believes in something, it's a law of physics within the universe. The only statements that can turn out wrong are lies, over- and understatements, and the like. If he genuinely believes it, it's definitely true.
You're right, I think it's great that characters can be mistaken. It's an important part of the HxH universe, and I also think it's great writing.
So yes, it's possible that Hisoka is mistaken. However, just because something is possible, doesn't mean it's likely. For example, Gon is an extremely optimistic and naive character. And, at the time that he fought Honzo, he was also completely lacking in combat experience. Thus, it's not surprising at all that he overestimates his speed and agility compared to Hanzo's. This is an internally consistent mistake for Gon to make.
On the other hand, the chairman has a vast amount of combat experience. He's been fighting life-or-death battles for decades and decades. Therefore, it's less likely for him to be mistaken that Pitou is stronger than him. And is he mistaken? I don't think so. There is zero evidence that the chairman's nen is stronger than Pitou's. If anything, the chairman acknowledges Pitou's strength by swatting him away in order to avoid a fight. And Pitou is completely unharmed by this swat, which is one of Netero's most powerful techniques. If Netero had said, "There's no way I can swat Pitou away," and then swatted him away, I would have called bullshit. But he didn't say that.
That brings us to Hisoka. Is it possible that he's underestimating his own speed, or overestimating the strength of the sun-and-moon explosions? Yes, but it's unlikely. Hisoka is an elite fighter with oodles of combat experience. He has also experienced the sun-and-moon explosions numerous times in the past few minutes. If he escapes the explosions next chapter without external help, I doubt it will be because he simply miscalculated. Based on what he said in this chapter ("I'm going to die anyway"), he is intending to do something suicidal. Escaping via bungee gum is not suicidal.
