Semantics refers specifically to meaning of words/language. If you say "it's only semantics" then it probably means you both understand and agree on the concept

Semantics refers specifically to meaning of words/language. If you say "it's only semantics" then it probably means you both understand and agree on the concepts but not the meaning of the words surrounding those concepts. That applies in this case, with the concept being breaking changes to a language along the lines of Python 2->3, and the terms being "version change" and "new language".