I think a PV "paint" could be a range extender but probably not a primary charger.
And that's sort of what the Mercedes article stated. In very sunny climates like LA, it might get you enough charge to be primary depending on your driving habits, but in most of the world it would just be a range extender.
The thing to remember is that while it can't give you enough range, quickly enough, to road trip on, it can charge while you're NOT driving.
I.e. let's say you get up and do your morning commute. Let's say you drive 12 miles in ~30 minutes. While you're driving, it obviously can't replace 12 miles of range in 30 minutes, especially not during morning hours when the sun isn't in an optimal charging location. But then you park it in full sunlight in your parking lot at work, and suddenly you now have 8+ hours for it to replace that 12 miles of range (and maybe then some). And then you drive it home at the end of the work day and burn 12 more miles of range. Is it that hard to envision a world where you've gained 18 miles of range while you've burned 24? If you're typically at a ~6 mile daily deficit, you can probably go a month or more without actively "charging" anywhere.
And note that the cost of that charging is... Free. Or, it's to say that you'll probably pay extra for the solar charging feature, so it'll depend on the payback period of "free" charging vs home or commercial charging. If you're only charging once a month, and you can do it on L2 or slower--i.e. cheaper than Superchargers--it could pay itself off quite quickly of the production cost of the feature is reasonable.
To me, this could be the killer app for people who live in apartments or don't have garages and have to rely on on-street parking. You've pointed out that the economics of an EV are VERY different depending on whether you can charge at home or not. The economics are also very different if you have a solar option to charge for free, even if it is only a range extender.
It remains to be seen if the economics work or if this is just a PR puff piece... It could be either. But IMHO, in theory even if it's "only" a range extender, it could be extremely useful if the economics work.