We built another object storage

So they built an object storage to replace filesystem.

And in "Why Not Just Use a Filesystem?", the answer they gave is "the line is already blurring" and "industry is converging".

The line maybe blurring but as mentioned is still a clear cut use case for file system - or if higher access speed is warranted, just slap more RAM to the system and cache them. It will still cost less even at current cost of RAM.

AIUI, one obvious difference between object storage and file system (beyond things like support for directories and file name lookups, which OP talks about already) is that an object storage has only atomic file store/replace, whereas a file system has to support arbitrary edits on both file content and directories/metadata.

Because (a) you have to mount a file system, so the user running the app needs permission to do that, and (b) It’s really hard to have a filesystem shared across untrusting admin domains.

With S3 you just do an http request and you’re done.

A lot of folks get hung up on the theoretical equivalence of things, and forget that their favorite solution may be flat out unworkable in practice for reasons that have nothing to do with the theoretical features they’re talking about.

Yes, so file system is a superset of object storage, making this even less of a competition. It's easy to implement object storage on FS vs. the other way around.

This is absolutely correct and the reason why S3 exist and are popular. However have you looked at the case being discussed here? There's a place for object storage and the pattern as discussed might even warrant being a cache or cache cluster.