There are two questions in play here:
1. When we ingest files or make new commits, how are these additions to the object store persisted?
2. When operations modify the working copy, how should these changes be reflected in the user's view of that working copy?
Ordinary git handles (2) by directly modifying the files on the filesystem. If you `git checkout` a branch, git will `rm` nonexistent files, `open()` and `write()` new ones, and adjust modification timestamps etc as needed. As you make changes to these files, some commands will occasionally "notice" that the file changed after the fact, and some may choose to modify the index to match.
the jj on github also does this, but inside Google, our concept of "working copy" needs to be disconnected from local files. Developers don't have their own local "working copy" backed by files on the ordinary filesystem; instead, we do all development inside a FUSE-mounted virtual FS called "client in the cloud" (CitC), so working anywhere inside our giant monorepo doesn't take any disk space (except caching). I think that's what the "Distributed file system" refers to - instead of modifying the local filesystem, jj would need to talk to whatever remote service provides the user's FUSE-backed view whenever the user uses `jj checkout` or some other jj operation that modifies the working copy.
When you speak of implementing IPFS storage, I think instead you want to keep the object store and operation log on IPFS while keeping the local working copy right on the ordinary file system, similar to how git-LFS keeps local files untouched while modifying the way they're persisted to the git object store.
Alternatively, perhaps we could imagine an IPFS backend similar to `jj git` and `jj native`, perhaps `jj ipfs push/pull`. Then, a completely local repository could push/pull to and from IPFS, completely agnostic of how the user's repository is stored on disk.
In any case, jujitsu's API surface is flexible enough to support any of these use cases since the author designed it from the ground up to smoothly support very different needs for internal and external users. Most users outside Google just want a familiar working copy containing ordinary files, and the fact that the repository structure happens to be backed by a git-like object store (linus' "git is just a merkle tree of files" philosophy) is incidental under the hood. That's just fine, even though most internal users will be interacting with a very different way of using jj when everything's said and done. Ideally, nobody needs to notice or care about the difference.
[1]: More about Google's internal VCS needs: https://cacm.acm.org/magazines/2016/7/204032-why-google-stor...
[2]: Linus Torvalds on git: “In many ways you can just see git as a filesystem — it’s content addressable, and it has a notion of versioning, but I really designed it coming at the problem from the viewpoint of a filesystem person (hey, kernels is what I do), and I actually have absolutely zero interest in creating a traditional SCM system.”