Is Gary right to be angry? Should you state the rating while playing in a simul? Isn't simul supposed to be a fun place where everyone gets to play their favorite player? Are there such rules in simul?
First off, Kasparov is not a big fan of losing. That's an important detail. I wont weigh in too much on his personality, but if you look at the comments on that video, there are quite a few strong opinions about what kind of person he is.
I don't know what the rules were for that simul, but if they were the same as the Kasparov's current website:
Players must be true amateurs who have 1999 ELO points or less, no exceptions. No player who has ever held a Rating of 2000 or over is permitted--it is contrary to the spirit of the simul and to the agreement. No coaching or computer assistance is permitted.
So if the organizers in Johannesburg were operating under these rules, then indeed they were out of line.
If you watch the whole thing, it looks like the under 2000 rule may not have been in place back then, but he did specifically ask the coordinators to tell him who the strongest player was. It appears they did not take into account a coach helping these players during the simul, and so they pointed elsewhere.
In some sense, it does make sense. At the GM level, you are always playing to your opponent. You have to know something about them. He was playing on assumptions about the capabilities of his opponent, and those assumptions turned out to be false.
On the other hand, you are right about this event being something that's supposed to be fun. I know I generally agree with it, but not everyone does. Kasparov does seem to care that a player can walk away claiming "I beat Kasparov" when Kasparov was not playing his best at the time. Instead, he was playing a version of Kasparov designed to do "something else," whether that's saving time for other boards or trying to teach the amateurs something about chess. We don't know what his purpose for being at the event was, but we do know he values his name and he values his record.