Wait, what? This might be a little off topic, but if you assert that they lack evidence and are drawing conclusions based on motivated reasoning, that seems highly relevant and not ad hominem. I guess it could be unnecessary, as you might try to focus exactly on their evidence, but it would seem reasonable to look at the evidence they present, and say "this is consistent with motivated reasoning, for example you describe many things that would happen by chance but nothing similar contradictory, so there seems to be some confirmation bias" etc.