I agree it's good to include "topic" / "post" in the button label, since Flow knows which one it is. But I still think "Add" is ambiguous. After opening the edit box and typing some text, the user has already added something to the board as they currently see it, and they might interpret an "add topic" button as "open another edit box for adding one more topic to what I already have".
The button's action is to send the addition the user created to the server, and the user shouldn't need to know that this is technically adding a new entry to the database rather than updating the stored version of the board.
One could also interpret "Add" as the whole sequence of opening the edit box, filling in text, and submitting it (this is how phabricator does it).