1) RAZ was bitten in 1091, and in the immediately subsequent strip 1092 it was blatantly obvious he had been infected and turned into Zombie Zilla. (If you count the obvious implication of Yuki's frantic "They are biting him!" text, and Yutaka's even more frantic "get away from him!" in response (neither of which was covered with any immediate attempt at humor), the lag in the reveal was exactly 0 strips. There was no series of subsequent strips in which RAZ appeared perfectly normal, Largo and Yuki talking as if she were perfectly normal, and a lone voice crying in the forumness "I bet she's a zombie!" proven right after a long interval, humorous or otherwise.
2) The joking you refer to was preceded by much distress and anguish on Yuki's part. It was blatantly clear from both the art and dialogue that Yuki was well aware that being a zombie was a Bad Thing and that having something as powerful and destructive as a Zilla in this state was far far worse. Narratively it also made perfect sense, Yuki states clearly that it's Yuki's fault, not the Zilla's; the obvious implication there is that she gets that her inexperience and Largo's cray-cray have not only caused harm to an innocent creature, but could potentially harm many more citizens of Tokyo if the creature can't be contained or killed.
Fred could have gone two ways here. (Well, at least two. ) Masamicihi could have been right, but still failed to persuade the fairer Sonodas, the monster could have escaped, killed or zombified large numbers of Tokyoans, much pain and suffering directly caused by Yuki and Largo biting off much more than they were actually able to chew. Yuki learns a hard lesson the hard way.
Alternatively, Fred could (and did yay) go much lighter; Meimi hints right away that steps can be taken, the minified Zombie Zilla becomes a merely household nuisance rather than national terror, Yuki still "learns her lesson" but nobody has to actually die for it. (Largo wasn't gonna learn anything anyway so that's a wash. )
Then there's the third possibility: Fred piles the jokes on strip after strip, people mostly go about their business for a while in various humorous ways, then suddenly the Zombie Zilla kills a bunch of people quite horribly. That's the plotline that is hypothetically equivalent to the one you're proposing, with the arrows (humor -> horror) pointing the opposite direction from the way things actually happened then (and, I claim, are happening now).
I would never discount an argument. I might offer counter-arguments intended to refute it, if I thought I had ones of sufficient relative weight, but I would never discount the original argument. For a scientist that would be like killing a puppy, "you could go to Science Hell for that" as Harry Potter once said.But, by all means, discount the words of this bear.