With all due respect …
Who laid the decking? Was it the OC or the builder when the place was built, or was it added on top of the existing surface as an option or by a subsequent owner? If the latter then I’d say it’s the responsibility of the owner.
If the decking was laid by a subsequent owner and fixed to the slab, and there is no by-law passing responsibility to the lot owner, then it’s more than likely the owners corporation’s responsibility, regardless of who installed it.
You are right, however about the lot owner being responsible for at least checking the drains.