I know some folk will defend anything DB says, but he clearly claimed the expansions were free, there is no amibiguity in that statement.
The ambiguity is in whether it was actually said that way and in that context.
An infinite number of things can go wrong when trying to translate a conversation into text.
Braben of course knows that Horizons is a paid expansion and that more of them are to be released, and it is nothing to be ashamed of either.
Lying about that makes no sense whatsoever, so there is most likely a simple explanation for that sentence in the interview.