I think you misunderstand. The statement "when running unrelated processes in userspace" probably means, when running multiple threads under the same process id

I think you misunderstand. The statement "when running unrelated processes in userspace" probably means, when running multiple threads under the same process id. Threads are "lightweight processes" in linux parlance (e.g. linux treats each "thread" as process, and then deals with the fact that some of them share memory). What they are saying is that this instruction is - for example - for a browser (a userspace process) to clear the branch prediction cache after running untrusted javascript on another thread.