I don't think it is, SoGent, but I might be wrong. I think that what we've called Givenchy Vetyver here over the last couple of years is the same thing as the mythical you mention. At least when I've said Givenchy Vetyver, I've meant the thing that has come in the tall vertical bottle and released a few years ago with several of the house's scents in similar bottles.
So I think they're the same. However, and this is a big however, this new-rerelease of Vetyver has a Givenchy Vetyver that came before it, sometime in the sixties or seventies, and until the recent rerelease was discontinued. If you mean that as the Givenchy Vetyver, and the rerelease as the mytical, then I can't say if they're alike because I've never smelled the old, discontinued, and possibly duplicated one. So I can't say if the new is at all like the 30-plus year old, but I would suspect that the recent rerelease is some reformulation. Can't testify for sure to that effect, but the others in the mythical release seem spot on to what they're labeled.
But I can say that there aren't two Givenchy Vetyvers in current release, one called Vetyver and the other with the name Mythical added to it.