I had this exact issue when installing these brackets in my AC/DC Luci VE a month ago. I can't believe nobody else mentioned it. I work in a machine shop that has a large selection of washers, so I just matched up washer thicknesses that were very close to, but not over the gap size and I tightened everything up with the washers in place. The corners of the cabinet haven't cracked yet and I'm quite the nudger, so I figure so far so good.

If I was to try to mount that bracket with the gap you have shown (and I experienced in my AC/DC), I figured I would strip the screws in the wood before the bracket ever bent out of shape enough to touch the side wall. I also did not want to take a chisel and scrape wood off the corner support to remover the gap, so washers it was.

Note: I checked the bend on my leg brackets and they were close to, but not perfectly 90 degrees. The wood corner block was just to thick to allow the flanges to sit flush with the sidewalls, exactly like the Funhouse pic above.