The only real difference seems to be in the kernel. And that one difference is that it's "capability-based", which in practice means that programs are given keys by the kernel, which they can use later to make syscalls. This theoretically makes security easier to reason about.
I think this is missing the forest for the trees: most of what would normally be a systemcall to the kernel in Linux is instead a capability-based IPC to other userspace processes with limited privilege. This includes device drivers, the networking stack, filesystems, etc.
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