Unless he completely made up the information he received, or lied about the attribution in the internal slack chat, he should not have been fired for this. This was an editorial error.
In fact, the exact information he passed along was factually correct by one interpretation. The S-300 rockets were "Russian" in the sense that they were Russian manufacturing. They were not "Russian" in the sense that they were fired by Russian armed forces (to the best of our knowledge).
Really think the AP has tried to pin the blame on this one reporter incorrectly.