The strata committee can’t make a decision like that. They also can’t have a valid meeting without issuing an agenda.
If it was an extraordinary general meeting, they should also have issued an agenda and invited you to the meeting.
I’m also not sure that they can change the strata plan with even a “super majority” or special resolution. Section 17 of the Strata Schemes Development Act says that the owner of a lot has to sign off on the conversion of that lot to common property. Don’t know if that applies to lot walls.
As others have said, they can issue an opinion about what is and isn’t common property, but that doesn’t change anything. It just lets you know what side they will be on in a dispute.
“Strataly” wasn’t a word before but it is now.