I wasn’t aware that Transformers The Movie was so important.
It’s very important, because it’s an anime.
Anime is an animated film/series that is originally made in Japanese language. That thing clearly does not pass the main condition.
A Google search said:
an·i·me
ˈanəˌmā/
noun
noun: anime
a style of Japanese film and television animation, typically aimed at adults as well as children.
Wikipedia says:
Anime (Japanese: アニメ) is Japanese hand-drawn or computer animation.
The Transformers along with its continuation, The Transformers: The Movie, was animated by Toei, a Japanese animation studio, and both look like any reasonably well animated anime from the 1980s.
Seems to me that it fits the criteria for being an anime pretty well.
I would say that being written for the English language originally doesn’t really hold much on the matter, especially considering that a Japanese dub was produced simultaneously, IIRC.