They need a basic remote API and SKD first. Now we have that anyone can implement these local daemons/translation layers if they want to. This is still early days.
They could at least have implemented a shell
They do have an interactive shells:
You can also run interactive shell. Just type ./proton-drive and then the commands directly.
Though the commands are not the same as Linux equivalents that is IMO not a big concern. Similar things like AWS S3, or even ftp also don't have the exact same commands. It is really a trivial thing to learn how it works.
And again, anyone is free to build on this now if they want to.
I would not be surprised to see a fuse fs popup at somepoint around this.