Because no genderswapping takes place when Ranma gets water poured on him, amirite? Genderswap doesn't refer only to non-canon genderswaps, though I can see how it might seem that way because Rule 63 implies that the genderswap is an unofficial thing.
The tag is described as "A "genderswap" refers to the almost magical version of a sex change, wherein a character of one gender turns into one of the other sex without any medical involvement."
