You raise an interesting question. If the splashback is directly fitted to a common wall, then it is obviously fulfilling the same function as ceramic tiles, and has been fitted in lieu of tiles, as you say by the builder. In the case of flooring, hard surfaces directly attached to concrete are accepted as being part of the common property, these may be either ceramic tiles or parquet for example, but not carpet. Same as vermiculite on ceilings. Quite frankly I don’t see any difference in an original permanent hard attachment to a vertical part of common property rather than a horizontal one, and if your property was constructed after 1974, then your Strata Manager may well be incorrect in his interpretation.