The Ch. 1 Shanks-side and Ch. 19 Buggy-side King-of-the-Pirates declarations were solo declarations — Luffy speaking the aspiration without comparison. The Ch. 60 chapter is the first comparative-claim against a rival captain — me, not you. The captain-vs-captain throne register is installed as a Luffy-signature here and recurs at every later major arc-villain captain (Crocodile at Alabasta, Doflamingo at Dressrosa, Big Mom at Whole Cake Island, Kaido at Onigashima, Blackbeard ongoing).
The chapter is structurally two King-of-the-Pirates splashes (pages 7 + 15) — Oda gives the comparative claim splash-shot weight, signaling its compositional importance.
Why this matters
This is the first on-page instance of the me-not-you throne-claim. Every later major arc-villain captain receives the same treatment from Luffy.