Is “unseductive” an established English word, or just coined?

The Oxford English Dictionary has two quotations using it in the entry on the prefix "un-":

"Nor upon that event did he think it necessary to fly the roof of two such unseductive innocent females as Mrs. Horton and her niece." -- E. Inchbald, Simple Story I. ii. 12, 1791

And the second:

"He looked at her neat unseductive clothes and thanked God she was like that." -- M. Borden, Black Virgin, ii. 26, 1937

So it's not unheard of, and definitely wasn't coined by the writer at Time. It's just rare.

As Will points out it is just rare, and in google's book corpus it appears since 1800s - see Books Ngram Viewer for unseductive.