Here are Apple-provided sources you can point at that show FSKit file system extensions are entitlement-gated and live in the "extension / system extension" world (not "just load a plugin"):
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FSKit overview (Apple docs) --- explicitly says you deliver an FSKit module as an app extension that runs in user space.
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FSKit Module entitlement (Apple docs) --- com.apple.developer.fskit.fsmodule is the entitlement that indicates an extension provides an FSKit filesystem (i.e., without it you're not "an FSKit filesystem extension" in the eyes of the OS).
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System Extension install entitlement (Apple docs) --- com.apple.developer.system-extension.install is the entitlement that allows a host app to activate/deactivate system extensions (the general mechanism used to get these kinds of extensions loaded/managed on macOS).
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WWDC statement about extension sandboxing --- Apple describes extension code being isolated into its own daemon process that's sandboxed (this