any of the expander (SATA, or SAS) things, yes, will be sharing bandwidth. but as you mention, it won't be a limiting factor for mechanical drives. and considering the latency involved in this sort of retrieval, probably isn't a problem regardless.

FWIW the break out cable they've listed is splitting up a connector that has 4 electrical channels onto 4 physically separate cables, so there's no problem with it. they just don't have anywhere to plug it in.