If you're wondering why they would count it even though it was a one off that was never picked up, whereas they could easily brush it off and do a new Eighth Doctor [which would be stupid] it wasn't just the movie.
Paul McGann became the new face of the Doctor, in the Books, in the DWM comic strip, and even the Audio Plays.
Had the show been revived immediately after the movie flopped, and they didn't decide to keep McGann, he may very well have been discounted, as was the case with the brief reign of the Ninth Doctor played by Richard E Grant.
Although, on the other hand, it was quite a big deal when McGann became the Eighth Doctor. In the papers and that, whereas REG was featured in a more low key animated episode, so even then its apples and oranges,