Fortran is older but it's array manipulation maps naturally to SIMD.

That's missing the point of the article, but it's also not true, at least not at the time of C89. This can be easily verified by a Google search. As an aside, Ada also had multi-threading support before C89 was released, but this article is about SIMD, not multi-threading.

c89 is 19 years into c's life

Yes, but before standardisation there were many implementations all doing different things based on a book with no particularly formal specifications, so picking 1989 makes more sense than 1970 in my opinion. Multi-threading also existed in 1970 but wasn't as widespread.

You had plenty of places outside Bell Labs doing concurrency stuff, Concurrent Pascal and Solo OS is one example among many.