Its actually rather nice of RTD to count the Grant Doctor. Surprising since I remember reading less than charitable remarks about his performance in Shalka.
Beyond that, there's no reason to not count him as middle-aged version of the War Doctor, or even a cancelled incarnation of the Doctor from the Time War where he regenerated from and then unregenerated back to John Hurt? All's possible.
Like I was trying to say its not Richard E Grant thats the problem, if anything I'm biased towards wanting him to be an incarnation of The Doctor. But to be fair I am treating that reveal as if I was neutral to the person in question, which leads to me having huge problems with the reveal as I stated.
Even ignoring the whole "The Doctor is less special if everyone is The Doctor" factor, it also screws with things in universe even more, assuming (like me) you're going by the lore of the show from its creation up until Chibnall decided to make it "his" show and not accepting his BS. Up to the Fugitive Doctor, if we add her in retroactively, we had a Doctor who (starting with the 1st Doctor) regenerated 12 times before getting more regenerations in The Time of the Doctor (assuming we no longer count the semi-regenerations like the 10 clone as having used up a regeneration), and this fits the lore of every Timelord being able to regenerate 12 times, as first stated in The Deadly Assassin. The timeless child could be completely retconned tomorrow and everyone up to the Fugitive Doctor still fits. But adding Grant ruins that, unless they pull some "he's an erased regeneration" thing or whatever but it makes no sense that some scanner would read that, especially since we've seen the "Doctor gets scanned and old lives are revealed" thing before and only ever saw legitimate incarnations.
Again thats just my feelings on it, in a vaccum I like Richard E Grant as a version of The Doctor and would even try to fit him into canon if he wasn't just the third secret Doctor we've had to deal with.