Not disagreeing this guys ‘touch grass’ is not helpful or useful, but it is also very clearly not ad hominem. Ad hominem is a logical fallacy and involves using an arguers characteristics to try and cast doubt on their logical argument. Its a fallacy because even if someone who doesn’t understand what they’re saying gives a logical argument the argument is no less correct for not being understood. Telling someone to touch grass is an imperative statement, and while hostile and rude, does not constitute ad hominem.