the proof is in the pudding with this one

It isn’t.

as you must also ask yourself why E15 is banned during summer months in the first place.

I did. And I shared that in my comment above.

Your source doesn’t share any data on the topic, even just as a summary, but it links to summertime smog, which links to “smog-causing pollutants”, which says:

Section 211(h)(1) of the Clean Air Act prohibits the sale of gasoline that has a Reid Vapor Pressure greater than 9.0 psi during the “high ozone season,” which runs from June 1 to September 15. (RVP is a measure of volatility; high-RVP gasolines release more volatile organic compounds into the troposphere where those VOCs contribute to ozone formation.) Gasoline-ethanol blends below E50 are more volatile than straight gasoline and cannot readily meet the 9.0 psi RVP requirement. Congress created a “one-pound waiver” at Section 211(h)(4) that increases the RVP limit from 9.0 psi to 10.0 psi, but—and here’s the catch—the waiver is only available to “fuel blends containing gasoline and 10 percent denatured anhydrous ethanol.” That is, only E10 can take advantage of the one-pound waiver. Although E15 is slightly less volatile than E10, its RVP still exceeds 9 psi. It needs a one-pound waiver to meet Section 211(h)’s RVP limit in the same way that E10 does, but it is not eligible for one under current law.

The article’s justification for why E15 isn’t legally permitted is that there’s a law against it, which is circular logic. From the environmental protection perspective, it doesn’t sound like there is data suggesting that E15 on its own is worse for the environment than E10. If the only argument is a legal one, it’s not a good argument.

If you can answer that question you'll likely find the information you're looking for.

I did, and I shared that answer in my comment above, too - but it’s not the answer you seem to think it is.