I am guessing here but I think this may be a misuse or very specific interpretation of the word “stratum” (which has several meanings in property law).
But putting this in the context of standard practice in strata law, it reads to me that it is defining the area of the balcony that is common property connected to this particular lot – from the surface of the slab to the bottom of the slab above. The fact that both slabs are also common property is neither here nor there.
Regarding the change of tiles, if they did so with permission (but without taking on responsibility for the tiles or the seal underneath) then the tiles continue to be the responsibility of the owners corp.
If they changed the tiles without permission then any claim they might make to repair or replace them in the future could be countered with a claim for restoring the original tiles.