They even said so at the end of the episode, IIRC. Not just about Q, but just that having seen that one possible future, and sharing that vision with the crew, he probably changed the future quite a bit as a result.
Edit: Ah here we go, from the transcript:
Spoilers for end of All Good Things
(the senior staff poker game has been going for a while)
CRUSHER: Take it.
RIKER: Any time, Doctor.
LAFORGE: Four hands in a row. How does he do it?
RIKER: I cheat. I’m kidding.
CRUSHER: You know, I was thinking about what the Captain told us about the future. About how we all changed and drifted apart. Why would he want to tell us what’s to come?
LAFORGE: Sure goes against everything we’ve heard about not polluting the time line, doesn’t it.
DATA: I believe, however, this situation is unique. Since the anomaly did not occur, there have already been changes in the way this time line is unfolding. The future we experience will undoubtedly be different from the one the Captain encountered.
RIKER: Maybe that’s why he told us. Knowing what happens in that future allows us to change things now, so that some things never happen.
(Worf and Riker look at each other)
RIKER: Come in.
TROI: Am I too late?
RIKER: Of course not. Pull up a chair.
TROI: What’s the game?
DATA: Five card draw, deuces wild.
(Picard enters, and everyone stares)
RIKER: Is there a problem, sir?
PICARD: No. I, er, I just thought that I might, erm, I might join you this evening. If there’s room.
RIKER: Of course. Have a seat.
DATA: Would you care to deal, sir?
PICARD: Oh, er, thank you, Mister Data. Actually, I used to be quite a card player in my youth, you know. I should have done this a long time ago.
TROI: You were always welcome.
PICARD: So. Five card stud, nothing wild, and the sky’s the limit.