I’m aware of another authoritative interpretation by the NSW Office of Fair Trading (OFT), who administers the Strata Schemes Management Act (SSMA) to the effect that a false ceiling was not a boundary of the Common Property, but rather the floor slab above it is that boundary and also the boundary of the cubic air space of the lot. That may be the interpretation that your Strata Manager’s relying upon.
One would expect common and consistent rulings as opposed to interpretations, but my personal experience gives me more confidence in the L&PI for matters to do with the Strata Plan (i.e. the physical drawing), the OFT is better with regard to matters involving the responsibility maintenance and repairs, and with the application of By-Laws.
I can’t wait for the Review of the SSMA to produce some Strata Legislation that’s less open to interpretations and more conducive to making informed decisions.
Good luck with convincing your Strata Manager though.