You don't directly say it, so maybe I'm reading you wrong, but you seem to be suggesting that this hypothesis is nonsense because autism existed before this preservative was used.

The article isn't saying that the only cause of autism is the preservative. It is suggesting maybe part of the increase is due to this preservative.

Although people died before cars existed, it doesn't mean that cars aren't the cause of some deaths today.

While propionic acid is used artificially as a preservative, it is a naturally occuring byproduct of fermentation of certain kinds of carbohydrates.

The article notes the link between certain kinds of bacteria in ASD children that overproduce PPA as a byproduct.