Section 7 of Schedule 2 of the Act says this:
Matters that must be included in notice of meetings
The notice of a meeting must include a detailed agenda for the meeting.
It can hardly be detailed if there are matters not on it. The purpose is so that owners can see what is about to be discussed and either attend or send responses. If they can gather enough votes from owners, they can even prevent matters from being discussed.
Any matter that was discssed that wasn’t on the agenda could therefore be challenged at a later date. The chair was right, regardless of the content in your motion.