Welly – it’s possible that the other two owners could have come up with some really convincing arguments that may have resulted in a different decision about the planter box, but they weren’t there to put their points of view because that wasn’t a Motion on the Meeting Agenda.
That’s why (in NSW) there’s no General Business permitted on the Agenda for General Meetings, and I’m afraid that your Strata Manager’s advice is correct.
If you want the planter box removed, then put that as a Motion on the Agenda for your Plan’s next General Meeting, or provided it’s not in any way fixed to the Common Property, then request your Executive Committee to consider it as a formal Motion on a published Agenda for their next Meeting.