Jack isn't really a bourbon

It checks all of the legal boxes to be a bourbon, at least 51% corn, aged in new charred oak barrels, the aging and proof requirements, made in the US, etc.

Being "made with a starter" isn't a requirement for bourbon, and I'm honestly not even too sure what you mean by that. I assume it's probably some reference to it being a sour mash whiskey, but that's not something that factors into the legal definition of bourbon.

The only thing that arguably makes it not a bourbon is the "Lincoln County Process" of charcoal filtering it before aging in the casks, which is a requirement to be called a "Tennessee Whiskey," all the other requirements are pretty much the same as bourbon (and it's worth noting that the 2nd biggest Tennessee Whiskey brand is Prichards, which is actually located in Lincoln County, and doesn't use that process and has a grandfathered exception to that requirement)

The main nitpick is whether that Lincoln county process can be considered to add color or flavoring, because if it does that would disqualify it from being a bourbon. I'm personally of the opinion that if it's a filtering process, it's probably removing flavor and color if anything so not a disqualifier, and even if it did, in the relatively short time it's in contact with the whiskey it's probably pretty insignificant and not gonna be all that distinguishable from what the charred oak barrels are going to impact to it over the next 2+ years.

And Tennessee is really the only place that makes the Lincoln County Process a requirement for "Tennessee Whiskey" Pretty much any other government or trade organization (like NAFTA) that has a definition for it basically just leaves it at something like "a straight bourbon whiskey made in Tennessee"