Zushi had Nen protecting his body and he's a scrub
Wasn't he in a tournament setting tho? Hisoka was just fooling around.
Btw I re-read the scenes in question. Regarding Hisoka it's interesting to note that the character Hunter applicant that was among them stated even if they had an army backing them up, they wouldn't be able to do anything. Even if you don't take that literally it's a point for not even many armed men functioning against that display of Hisoka. Hisoka was fully focused on Leorio and Gon had the time to prepare the attack, stretch the distance and use it to its full capabilities. Hisoka's head was moved somewhat but he didn't lose balance or anything, and only a little blood was drawn from besides his eyes. He still only noted Gon's weapon as creative and wanted to have a better look at him. It was when looking him in the eyes that he deemed him as a worthy Hunter. It's not that big deal imo. Uvogin had his head budged by a not expected bullet and even tho it wasn't as much as Hisoka's, it still hurt him. And still when guarding and expecting, he could protect himself against a Bazooka's attack, many times more powerful, and it had barely an effect over him. It was then the regular humans realized they were totally screwed for messing with Uvogin, possibly meaning that Hisoka can defend and guard himself against wayyy more powerful attacks if he's actually expecting it. A similar concept in other series.
As for Netero, Gon's loosening boots had an effect on him, which isn't a strong attack at all given its nature, it was only meant to hit a Netero that thought he had dodged it, and Killua could kick him from behind, but then Netero changed his gaze and used more speed to get the ball. When seeing the speed and the force to which Netero used to kick the ground, Killua realized they could be years at it and they wouldn't get the ball, meaning Netero was just messing around a lot and that them almost getting it was against a totally fooling around Netero. He was as casual as ever. He wasn't intense.
I don't think Nen was thought up back then, but I don't necessarily think those examples show an obvious retcon of their strength. Other ones such as Hisoka bleeding in his fight against Kurapika and such, maybe.