Multi-value registers are CRDTs for sure. Conflict-free doesn't mean that values can't have concurrent histories (or, as you say, "multiple versions") -- it means that the merge operation always succeeds.

What's the definition of a conflict, then? Equations welcome.

Feel free to read up on CRDTs; I'm confident this will answer your question.

The short answer is, roughly, that a conflict is a discrepancy in state which cannot be mechanically resolved.

I've read a few CRDT papers. Perhaps you could name a specific one.