That seems like a very realistic/feasible figure. (Lightyear One's claims their prototype car can pull almost 2kw per hour (7.5 miles of charge per hour), which at 6 hours, earns you 12kWh per sunlight-day)
It’s not “.7kW per hour”.
Watts is a measure of power, it’s already Joules/second.
You just want “.7kW”.
For 6 hours, you still won’t get 4kWh, because you need to adjust for less than ideal angle, and less than full sun days.
But if you got 3kWh on a good day, that’s still 10-12 miles of additional range.
Car environments tend to be worse than that due to the angle and likely regular proximity of even mild shading. Its certainly enough to eliminate vampire drain, which is helpful at times, but I don't see how it will add an appreciable amount of mileage otherwise.
An EV uses ~250wh/mile. This varies a bit for some, but none of them are going to come close to 20 miles in .7kwh. Maybe 3-4 at most.
The ability to use something like this when left parked at an airport would be nice, though. Though I suspect that readily available 120V outlets is actually a cheaper option in the long run, relative to deploying panels on every car.