If you read what it says, they've changed an implementation of a function which may change ordering of unordered collections, and that may break some people's code if they relied on some specific ordering. They are not talking about breaking interfaces. In fact, they specifically say they are not aware of any binary incompatibilities. Their "strive to maintain" works out very well in practice, so far.
I wrote that sentence and you should take it as written. In general, we strive to maintain binary compatibility (old AOT-compiled Clojure code should continue to run on newer version), we do not exhaustively test for or guarantee that. At times we have broken this in alphas etc and we take it as a high priority to fix such things.
