Pardon the expression, but this is "Trump Logic". Just because series that were produced BEFORE Enterprise existed don't reference Enterprise doesn't mean that Enterprise is suddenly relegated to occurring in an entirely separate timeline that itself didn't exist at the time Enterprise was produced.
I'm no fan of Trump. But what you've exactly stated is why it makes it so easy to fit ENT in the Kelvin timeline. With really no reference to ENT in the shows that were produced prior to ENT there is nothing preventing anyone from placing it in the Kelvin timeline especially when the Kelvin timeline is the only timeline that actually does reference it.