If he's going to build it over farmland, whether he uses solar panels or not is irrelevant to the farm. The land would be rented/bought/imminent domained just like normal rail would.
He makes the point that he doesn't need to block 100 feet from each side of the track, as well as put barriers in place to prevent animal crossing and blocking farmers from getting to the other side of the farm, in the same way power lines today use space. That means he would need some non-farmable area around the track, yes, but that it wouldn't disrupt the farm like a normal HSR track would.
I've driven from LA to SF and back along I-5 a few times, I recall lots of cattle farms, I don't recall ass much agriculture (though their definitely was some).
At the same time, the aquaduct runs along I-5 for a considerable length. I think building along the aquaduct would make a lot of sense. I'm assuming that land is already owned by the gov't, and is already somewhat protected along it's length.
I couldn't find the aquaduct on google maps, but I do see lots of "brown" land, so I guess it's not as heavily farmed as I thought. Not enough water presumably?
He seems to be planning to build this over farmland? Because farmers aren't going to be happy about someone blocking their light.
Light is the limiting factor in plant growth.
If he's going to block the light to farms he'll have to pay them for the lost crops, and then he might as well just buy the land.