Also, the term “tanna upalig” is used in the gemara for rav and othet 1st Generation amoraim (though almost always rav) that he can argue with tannaim because he was a tanna himself, but he eas also an amorah who other amoraim argued with, so even in the gemaras expression, he was a half tanna, half amora.
What this means for the baal hatanya is that he is fit to argue on previous generations, perhaps he can argue with the Gaon! But that doesn’t make him an actual tanna; gedolim are sometimes called tannaim as a praise(actually , i saw a letter from someone calling the Gaon ‘hatanna ha’eloki’ in the same vein)but not literally. By the gaon, they said that his neshoma literally belonged to tekufas harishonim