POSIX isn't underspecified here. rename() has no durability guarantees. That's the answer. "something that worked on FFS didn't work on ext4," As the article

POSIX isn't underspecified here. rename() has no durability guarantees. That's the answer. "something that worked on FFS didn't work on ext4," As the article notes this is NOT durable on FFS. Early FFS (and early ext) could lose the entire filesystem after a crash. ext2 still can. ext4 is the oddball here. TFA is not framing things correctly.