Sure, you don't technically NEED to have done that, but in Fate/Stay Night there is no reference at all to him being one (can't say about the other routs), and besides, a character of Archer's caliber deserves better then 'got in on a technicality'. And anyway, there is quite clearly going to be a future choke full of opportunities for Shirou to become one, if this school is anything to go by. We have the perfect setting, and archetypes abound, the very air thrums with mythic tension. Why settle for a hand-wave when I can justify the real thing?Originally Posted by The Type Moon Wiki
Of course, Archer is trying to prevent his glorious destiny from coming to pass. Hence hunting down the subjects who began all this, and trying to kill them, since killing Shirou probably won't work. Unfortunately, they are all under protection, he won't make much headway, and shortly he'll realize he's just done exactly the same thing that Archer did in his lifetime. He will then start second guessing himself, and so on.