This shows the brilliance of ONE once more because he trolled us and though we knew it was coming we simply didn't know how.
The off-screening of "that man" after all the build up accomplished two things: first the trolling of course and second it also allowed him to keep the gag sub-arc of Flashy thinking of making Saitama a disciple and Sonic still having hope that he can still beat him someday. I bet if they met "the man" and felt his bloodlust and presence then seeing Saitama one-shot him would make both of them give up their delusions and plans for Saitama. Personally I love this web comic more than the mainstream manga and anime, I think the crude drawing even adds more to the storyline.
I sure hope Murata draws a fight scene between "him" and Saitama which is basically "him" dashing around and attacking Saitama while Saitama tries to get him to stop with words, gets frustrated "he" won't stop then 1 shots him 6 feet under. I imagine that his "gun" is like snakes sleep dart gun and tasers aren't loud.
I should have expected that with all of the "that man" build up. Two reunited rivals at the end of a dramatic arc going to face down their final foe? Of course Saitama has swatted him already.
@Liquidxlax
Basically it would be a repeat of Saitama VS Sonic. And One probably doesn't want to keep "That man" around because he is unnecessary for the story. So no wonder that he discarded him off screen.