Zeff is the first named-on-page Grand Line survivor present-in-scene in the series. His backstory is delivered via a villain naming the legend (Krieg recounting Zeff’s reputation) — Oda’s mentor-by-introduction-via-villain grammar. The legend is laid in before Zeff has acted on-page in any pirate-related way, establishing his identity as already known to the reader.
The seed predicts:
The lost-leg circumstance will be revealed via backstory chapters (Ch. 56-58 per series-chronology convention).
The Zeff-Sanji bond will be revealed to be deeper than mentor-apprentice (page 13 Zeff’s apology to Sanji + page 4 Sanji’s silent listen to Zeff’s hands-vow already imply this).