You got an answer.
Let me ask you, would you ask your plumber for an assesment of that pain you have in your back?
Local councils are completely unfamiliar with strata, and especially your particular strata. It’s unlikely they gave an informed and legally robust answer.
As others have asked, post your strata plan on the forum for at least a more informed answer, or speak with a strata lawyer.
If you assume it’s part of your lot, and you change it, and it turns out to be common property, you have just broken a bylaw.
On the other hand, if it’s your property but the OC spends money on it, you could be asked to repay.