Gonna have to call BS on that as the sharingans present on the following page of the scan I posted would point to it being Genjutsu also that statement only proves that him being punched wasn't a part of an illusion not that everything that took place beforehand wasn't one
Edit: Actually you may be right , as they're falling Kishi makes a point to show that Deidara looks Sasuke in the eyes, but Deidara closes the right eye and looks with his with his trained left one:
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So either Sasuke beat Deidara's left eye or Deidara see's there isn't a genjutsu
IMO the Sharingans kishi puts here soon after indicate Deidara's eyes being bested,similar to his eyes being bested when he faces itachi
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