That's just what Kaidou said, and not even what Kaidou believes in, since he didn't turn off his zoan form nor threw away his war club while saying that.
And even supposing that haki is really the most important factor to determine someone's strength, there's still no confirmation that conqueror haki necessarily transcends the other two kinds of haki.
Supposing that character A has:
Conqueror haki: 50 points
Armament haki: 50 points
Observation haki: 50 points
And that character B has:
Armament haki: 100 points
Observation haki: 100 points
Character B's haki is superior to character A's, even though he doesn't have conqueror haki.