My take on it is this, people have built amazing physiques for decades without ever worrying about a Zercher squat. You can do them to your heart's content and I am the type of person to avoid ever stepping in to give anyone advice unless they have already removed their head from their shoulders and it is rolling across the gym floor towards another lifter. In that case, my advice would be, "look out", but that's beside the point.
He obviously felt they were useless in terms of strength or size gains. You may disagree with that. One thing I can guarantee, however, is had you appeared to actually know what you are doing, there is much less of a chance that anyone will try to correct you on it.
That doesn't mean you tell him to "fuck off" just because of this. It does mean that in the "unwritten rules of the gym" your progress determines how people respond to you. Deal with it and make more progress. Everyone else had to.