There are multiple pull outs and established dispersed camping opportunities all along the entire road through the Ancient Lakes area. The specific spot marked on the dyrt map is just one spot. But consider this a review for the whole area.
The whole area is very unique, and the highlight is definitely hiking down into the canyon and Dusty Lake area. Spider web of trails down there open for hikers, mountain bikers, and horses. Overnight backpacking is allowed down there as well.
The upper lakes offer fishing and boating, paddle boarding as well if you are confident in your skill to stay on the board. I would not recommend swimming, or getting in the water at all, as it's all agricultural runoff.
The spot I stayed is south of the marker on the map, right on the north west bank of the Evergreen Reservoir. This was early April, on a Saturday, very late afternoon after hiking all day. Most spots were already taken, and I snagged one of the last established spots I could find. More of a pull off, pretty close to the road, and on the smaller side. But there are other, larger, spots if you can grab one.
Essentially no shade, very exposed, spring and fall would be the best time here.