Did he really think that Sevan would use bologna by choice, or that Marr would have forgotten binders for her crabcake?
Being a judge on a show where you know there is sabotage but they're not allowed to admit what kind, yeah, I'd probably assume that was what happened too, but then at the same time you have occasions like the first one with the chef with the not-enough-lobster and the radish "salad" that was so not a side. That chef just botched the round, but if you're judging and trying to guess how they were hindered in that round, you might think she had ingredients stolen and was only left with said radishes, or something.
The point of not being allowed to tell him is so he is just judging the dishes at face value, not in the context of "so and so had a harder penalty". In Simon's case, he's judged enough of these sorts of shows and seen poorly though out dishes enough that he probably knows for every wacky dish that seems like you could guess the sabotage, it might just be that the chef was an idiot. So you give them the criticism straight up, and if it's something stupid they actually did, hopefully they learn from it, and if it were just an auction item, they don't take it too badly.