Jamaal `Ataaya makes an uncharacteristically weak point about this claim, in his refutation of it in his book حقيقة النسخ وطلاقة النص في القرآن, pages 189-190. He says that the prohibition against fighting in the sacred months is for Muslims amongst themselves only, but for their enemies no such sanction. He brings for evidence,
I don't know how he missed the words "Except those you covenanted at the Sacrosanct Mosque, as long as they are straight with you be straight with them"?! How can that be evidence that fighting non-Muslims can be during the sacred months?
Then he quotes for evidence,
I don't know how he missed the word "If" which starts the verse?!
Why has Chapter 9 been so hard for so many Muslims to understand? God makes the same point over and over in this Chapter but not too many got it. The point is: fighting is for self defense and retaliation only, never preemptive or aggressive. Treaties must be honored. There shall be no more armistice treaties with polytheists. Without a non-aggression treaty, polytheists have two options: live peacefully with Muslims or else their aggression will be met with a fight.