The kids movie featuring zombies had it wrong
First of all, at the time the movie takes place in, zombies were not that much of a trend as they are now.
So I kinda doubt the kids would have even thought of having zombies in their movie.
Zombies were nowhere near as popular or in pop culture as they are now.
Second, zombies were still thought of as beings that had risen from the dead. Not as someone alive who had mutated.
And that is how the zombies were explained in the kid's movie.
So they wouldn't have been the result of a chemical plant.
Damn, I'm good.