Sure its a possibility there are 10+ GCH on a flight but what's the probability of them all going for the vegetarian option too? You would fairly presume most GCH who had a dietry requirement would have pre-ordered a special meal as a default in their prefrences. You're right the probablility is more than zero but I do think it's very slim.
Your advice is correct, we should mention it asap to aviod the problematic situation.