Cluster mempool definitions & theory

I’m a bit confused here; could you elaborate on how the gathering and stripping theorems are used here? Since the gathering theorem applies when a sublinearization is moved to the front, it alone doesn’t prove that L_{opt}' \gtrsim L_{opt} since L[p] is the ordering from L rather than from L_{opt}. I’m not sure where the prefix stripping theorem comes in, so I must be missing something.