According to the SE chat faq:
Rooms will exist indefinitely, so long as there is at least one person actively talking in the room. ... Rooms not worth retaining which are inactive for 7 days will be deleted. Rooms worth retaining which are inactive for 14 days will be frozen.
I wonder if the room is considered inactive if there are messages in the room posted from feeds (not from users).
My guess is that messages from feeds did not influence freezing/deletion. One reason why I think this is the formulation "there is at least one person actively talking in the room" from the faq. Also when looking at list of rooms for some site and also at the room info, the last activity shown there is the time of the last message shown by actual user not the last message from the feed. (I have on purpose put here a link to a room which is not very active, so it is possible to observe this thing there. Although at this very moment the last messages in that room do not come from the feeds, but from users.)
So I am pretty confident that the messages from feeds do not count for the purposes of chatroom pruning. But since I did not find a place in documentation/on meta where this was clearly stated, I decided to ask this also here.