Based on user research, many users do not understand that the word
"Active" means "recently created or edited."
I appreciate the fact that you are responding to user research - however, I fear that "Modified" may not be much clearer than "Active" in case a post is new and not yet edited (or old and never edited).
I interpret "Modified" as a change in an existing post, therefore this wording makes me expect an edit that is not there for not (yet) edited posts, i.e., Modified? What/where/how? I don't see any modifications! In contrast, the previous wording "Active" does not have such a strong "change" meaning.
You could say that this is a modification from the state of not existing to the state of existing - but that would be kind of a stretch IMO.
So I propose to either not show the field at all for never edited posts, or to write "Modified: never" instead of "Modified: today" in such cases.