So is it safe to assume that the term was coined _after_ the film's initial release, as a way of spicing up the ad copy for re-release?

My interpretation of the term runs more along the lines of
"alien shapechanger;" xeno = foreign or alien; morph = to shift or change shape (the latter word being a bastardization of the word "metamorphosis"). Given the actual method of alien takeover in the film, maybe "alien shape assumer" is more literally correct.

... Reed