When the Pope speaks (a) to the whole church; (b) in his supreme apostolic authority; (c) definitively teaching; (c) a doctrine concerning faith and morals. All four conditions must be present. (This was defined by Vatican I. Look it up; the name of the document is Pastor Aeternus.)
An interview given to a secular journalist flunks a, b, and c, hands down, no questions asked.
erratum
(d) a doctrine concerning faith and morals.