But the article itself addresses only the context of ATACMS. Not whether the US is "dictating what can and cannot happen in the war" generally.

Either way -- according the definition in Wikipedia, it is a proxy because one side is strongly supported by an external power. Sounds reasonable, and I can go with it (on at least a technical basis).

Where people go wrong (not saying you here) is when they accept the term "proxy war" and assume (or insinuate) that it means or supports the idea that Ukraine is simply a puppet state, not really fighting out of its own motivations.