Checking how the word "king" appears in Japanese, Conqueror's Haki is of course Haล-shoku no Haki, with the word Haล (่ฆ็) meaning supreme king. In King's question he uses only the word "ล" (็) meaning just "king". King's own name, btw, is written ใญใณใฐ (Kingu), in English and not in Japanese.
I believe King meant his question to be: So you have Conqueror's Haki now, huh?
Though King having no Conqueror's Haki himself makes me wonder why Kaido named him King. Did he believe he could reach that level?