Zac didn’t intend for Starfleet to notice the delta. He was content to just stay on Rigel VII as High Lord Zacarias, thinking that Starfleet would never return to the planet because of the debris field and the radiation. But then the Kalar used the delta as a symbol and it got spotted.
PIKE: Zac. We saw your message, the, um… the Delta in the garden. It’s why we came. Isn’t that why you did it?
ZAC: The people here adopted it as my symbol. I should have known better. It’s all getting torn out tomorrow.