Technical requests are usually handled by different people than catering and hospitality requests.

The people in charge catering and hospitality would not be expected to read the technical requests to do with electronic and sound systems, and vice versa.

The guy at the TRC podcast actually searched out their contracts, and the brown M&M clause isn't in the middle of technical requirements, but is in the hospitality section among requests of meals, beverages and dressing room requirements, and there's no mention that the concert would be cancelled if brown M&Ms were found.

So seems unlikely to be true, and just be the guy from Van Halen telling a fun story.