.. i dont work for envion. but i would guess they just use regular old electricity at night.
also solar is a small part of the business right. It will be a cost benefit analysis using their new system they have in place that locates cheapest prices overall, which may or may not be solar. they are also providing heat for locations using the miner and being paid for that to offset energy cost. Also remember that electricity being found cheaply is just icing on the cake. its very profitable without doing that.
I agree that electricity from the grid would make the most sense, at least until battery costs come down more.
Also, your other points are good ones. I've been mining for a little bit with both Hashflare as well as a home miner, and it does seem that it can be profitable even without super cheap electricity. Of course, the big unknown is how long that will remain true as difficulty levels increase.