It has to be remembered that when writing Zoro’s flashback, Oda was 22 years old, on his first weekly comic, and I think would have been only two weeks into life as a weekly author- I think the first three chapters of a manga are generally written before a new series is published, Zoro’s flashback was chapter four or five, can’t quite remember which.
So it being messy and not quite what we’re used to later is only to be expected. It’s not until the manga had been running for over a year that he did a multi-chapter flashback with Sanji. It’s entirely possible his editor wouldn’t let him do big flashbacks until then, or Oda wasn’t comfortable trying them, or he simply hadn’t hit his writing stride yet.
So it being messy and not quite what we’re used to later is only to be expected. It’s not until the manga had been running for over a year that he did a multi-chapter flashback with Sanji. It’s entirely possible his editor wouldn’t let him do big flashbacks until then, or Oda wasn’t comfortable trying them, or he simply hadn’t hit his writing stride yet.