Contact patch and rubber compound being close to constant when compared to a pneumatic tire the mileage impact would primarily depend on weight. I don't think it will be much of an impact since Michilin is unlikely to make a very heavy tire as that would have bad NVH characteristics which don't jive at the high end and reading between the lines of the press release stuff seems to indicate that's where they're targeting.

My guess is bad, and they’re destined for riot-control vehicles or all-terrain vehicles where mileage doesn’t matter as much as other concerns.

Adjusting inflation pressure is a feature. I run my tires higher than the vehicle recommendation but still below the sidewall limit for improved fuel economy. Airing down for sand is a thing too.