Words mean whatever the people using them use them to mean; they can evolve as their usage changes, etc.
Jargon terms are defined, once, in some journal paper or textbook or standards document; and then anyone who uses the jargon term thinking it means anything other than that original definition, is wrong. Jargon terms are prescriptivist by definition — that's the point of them, the thing that make them useful, the reason people invent them.
C89 is a jargon term. It it is an abbreviation of "ANSI X3.159-1989", the name of an immutable, public-accessible document written back in 1989.
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That being said: although "C89" itself has an exact jargon meaning, these aren't well-defined jargon terms:
1. what it means for something to be a "C89-supporting compiler"
2. what it means when a piece of code says that it requires only a "C89-supporting compiler"
However, given a common-sense understanding of these concepts as they would be defined to be useful to a working programmer or OS distro library packager, I would expect the following interpretations of these concepts:
1. a "C89-supporting compiler" is a compiler that supports a superset of the C89 standard, such that it can at least compile code that conforms to the C89 standard, to object code that has semantics that conform to the C89 standard. Such a compiler MAY (in the RFC sense of "MAY") be able to compile other code as well — perhaps with effect defined in some other standard; or perhaps with effect defined by no standard at all.) This is an "at least" guarantee — a "C89-supporting compiler" supports at least C89.
2. code that states that it requires a "C89-supporting compiler" (with no other specified requirements), should be able to be compiled by a hypothetical compiler that supports exactly and only the features and translational semantics defined by C89 standard. You should be able to implement a new compiler from scratch, to the C89 spec, and then compile this code using it. The code should be portable to any arbitrary compiler that claims to implement C89. This is an "at most" guarantee — such code will invoke features such that it requires the compiler to at most have 100% adherence to the C89 standard.