Looks like they have their own API called FileProvider:
Look like Seafile Ltd. is aware of it and possibly bought a license.
The change is not affecting vfs drivers at the moment. So OSXFuse is not affected yet. We’ll follow this with the OSXFuse author.
We also noticed this API. We haven’t tried that. But from what we heard on the internet it’s not actually working or released yet. It’s also limited to the Finder itself, not a virtual filesystem driver. So you cannot access the file from terminal. We’ll follow the development of the kernel extension API change and this new API.
BTW we did buy a license from the OSXFuse author.