.. and vice versa?

Not necessarily. The line of thinking in the essay I pulled it from assumes that every artist has a message, and that that message informs his work, so any given work attempts to convey that point of view to the viewer. While propaganda in and of itself is not inherently art, I would say that the most effective propaganda is art. The Great Dictator? Triumph des Willens? Battleship Potemkin? J.M Flagg's "I Want You" poster? Shepard Fairey's "Hope" poster? Any given Soviet modernist poster chosen at random? Certainly possible, but certainly not guaranteed.

Plus it's really punchy and fun to say :)

Is expression an agenda?

In theory, propaganda is imagery for a purpose that it serves, where true art has no such purpose. In reality, there's a grey area the size of Plymouth, which itself, is a grey area, the size of Plymouth.