The brackets there are mounted backwards; the long end should go against the wall. When they rate the brackets to hold 1000 pounds, it's based on a load at the center of the short side. But really the risk comes from the mounting. If you mount a 30-inch-deep hunk of desk on it there is a much higher stress on the mounting points since the long desk acts as a very effective lever. If you lean against the edge of the desk it may well pull the bracket out of the wall--especially if you use typical screws rather than lag bolts to mount it to the wall.
If you are mounting something like this in a corner, as being done here, the whole setup could be strengthened by mounting a ledger board against the right-hand wall to support the entire depth of the desk. Really you should have a support on the front-left corner as well, which can be done either with a dowel or furniture leg (you can get either at a hardware store).