Acedemics with an ax to grind aren’t worth anyone’s attention. Gedolim, who had Kol Hatorah with one scan, were very well aware of Leshonos that were popular.
Like the Gemara, there are additions from later times. The Kamarna says that Raya Mehimna was written in the time of the Geonim. Many Midrashim are from then, as well.
One example of their great Kashos is that the Zohar Hakadosh uses the term, Af Al Gav, while the Gemara uses Af Al Pi, and supposedly, Af Al Gav only shows up in later liturature. When I heard of this Kasha, the first thing that came to mind is that I bet they looked at Bavli, while Most Midrashim were written in the Yerushalmi dialect, as Rashi writes. A simple Google shows many Af Al Gavs in Yerushalmi.
But, as pointed out, there are certain words or sentences that staryed out as someone’s marks and made it into the text. Many Gedolim have said this, including the Yaavetz and the Kamarna.
Reb Aviad Sar Shalom, in his Emunas Chachamim, writes that even if Reb Moshe de Leon would tell him to his face that he wrote the Zohar himself, he would not believe him. There is especially no room for questions after the Arizal, who understood its full depth, testified as to its greatness and origin.
Anyone who is lucky enough to have some insight into its depth will not comprehend the notion that a certain 14th cetury Mekubal fabricated it. It is not a tale, it is deep and profound. Tzadikim and Gedolim spent their lives understanding its wisdom and reached great levels through it.