I am curious about that myself, I know very little about apfs.

But Apple has historically been strong on organizing and supporting teams (see: their chip design), a filesystem sounds exactly like something they'd do well if they decided to give it the proper investment and support.

Where they seem to be falling down these days is software maintenance - many, many reports of MacOS getting buggier with every release. But a big, complicated, but well defined and self contained engineering project? That's their ballpark.

APFS was the third filesystem designed by Dominic Giampaolo and fourth that he'd worked on, had a full team working on it, and had absurd testing resources thrown at it. It was set up to succeed in every way that a software project can be.