There's no source. That's more BS rumor-mill fodder.
Harley, Victory, Yamaha/Star, Indian and a host of other MFGs are still making plenty of air-cooled engines and doing just fine within the regs.
I wish this rumor would die already.
And as a side note, some air-cooled engines *are* actually at the cusp of failing EPA regs...
... for SOUND emissions, not hydrocarbons. Building the engines loose and rattle-canny for air cooling makes them loud and clattery. To the best of my knowledge, every air-cooled engine in the big boys' lineups are well within CARB regs.
You're right! Water cooling is NOT for Hydrocarbon emissions, it is for NOX emissions, which are very hard to control without temperature control. And it is NOX emissions that keep getting much lower compared to HC reductions. As a retired emissions enginer I do understand that NOX and combustion temperature is the limitation. That is why Harley seem to think just cooling the heads will work, and it will for a few more years. You are correct that a water jacket also aids in noise passby standards.
I love this Honda!! I was excited when I saw it several years ago, and disappointed when they didn't bring it in then..
For the right price, I may yet get a chance to have one!