From what i remember of the Translation of the game Kiritsugu introduced himself as a Sorcerer. to be a Sorcerer recognized by the Magic Association one need to have wielded one of the True Magics.
In Tsukihime, Aoko introduces herself as a Sorceress which she is because she wielded the 5th, however at the time of the original Fate/Stay Night release the Nasuverse wasn't as defined as it was today. While the game explains Thaumaturgy, Magecraft and True Magic (including the difference between Medea's and Rin's Magic) a lot of the information and inner workings of the Nasuverse (along with terminology) comes from the Fate/Complete Works Books, so at the time a Sorcerer/Sorceress was probably just a term to make a Magus seem more important before a majority of the True Magics were defined.
Also, in the wiki on Magecraft it says:
However, modern Magecraft remains far inferior from that of the past, so much that someone like Medea, who comes from the Age of Gods when Magecraft was much more powerful, would seem like a sorcerer to the eyes of a modern magus, even though what she can do is not really Magic.
Taking that into example, Shirou, a child who knew nothing of Magecraft would see Kiritsugu, who does use it, in the same way a modem Magus would see a Magus from the the Age of Gods.
Also, rather than having to explain the difference between Magic and Magecraft later on Kiritsugu probably figured it be easier if he had just said he was a wizard, since both a sorcerer and a wizard are "users of magic" this probably in japanese would be mahoutsukai either way. This is another reason why Rin disliked how Kiritsugu trained Shirou (even though Kiritsugu didn't actually do anything), because Shirou doesn't know the difference.
Another reason for all of this however as stated by Krazer in the comments is that we see the scene of Shirou though Shirou remembering the events of when he met Kiritsugu (we can tell this though the use of descriptive text of how Shiro feels). Shiro's idealized concept of Kiritsugu may have influence the scene, this would ring true of Shiki in Tsukihime aswell when he first met Aoko and he "remembers" Aoko refer to herself as a Sorceress when they first met when he was a child (and in hospital much like Shirou) while in the Eclipse Ending Shiki makes no mention of Aoko being a Sorceress.