Their favorite is: There is no credible proof that this actually happened.
And of course they are the arbiters of what is credible and what isn't.
The neocons' other main strategy is to say that anyone who opposes intervention on the side of the rebels is just a flunky of Assad.
The funny thing is, nobody here advocates intervention on the side of Assad, nor does anyone deny that Assad has committed atrocities. In contrast, the advocates of intervention either ignore or dismiss any and all reports of atrocities committed by rebels, and actively support sending US troops to fight for them. So who is the "flunky"?
In what way are stories like this one less "credible" than accounts of Assad using poison gas? Since neither the neocons nor their opponents were eye-witnesses to either, you'd think that they would be equally credible (or incredible).
In fact, "credible" just means "serving the interests of our agitprop."