I think the point can be summed up more easily.
Imagine a tobacco representative comes up to you and says cigarettes were recently discovered to cause white teeth and fresh breath, as well as fight cancer. He also says anything you can show him to the contrary is part of a gigantic conspiracy.
Not only is this person clearly lying, he is in a position that would be greatly advantageous if people were to believe his statements. Are you going to have an honest, drawn-out discussion with that person citing sources against each of his claims individually while he simply rejects them outright, or are you going to say "You're full of shit," and walk away? I'd go with the latter.
Sure it's an ad hominem, but it's not unwise or unjustified.