Semantics, yes. But this is a semantic discussion, so it should go without saying.
Opinion, no. People choose to use specific words to convey a specific meaning. This is why language exists. The term "Dracthyr" exists to differentiate the Dracthyr from Dragons. If they were the same thing, the term "Dracthyr" would not exist.
They have. They've also applied the word to cloud serpents, faerie dragons borrow the term, and chimaeras/rylaks have also been called dragons, among others I assume. But they aren't Dragons. If they're as synonymous as you claim, the term "Dracthyr" is redundant and doesn't need to exist.