I think you might be understanding the term ‘hub’ to mean an external device plugged into a USB port to provide more ports. I’m pretty sure Quartz is talking about (or including) root hubs - of which there may be one, or several, built into a motherboard, or on installed PCI cards. There is a maximum current supply rating per hub in this context - and it’s this that often imposes the limits.
Absolutely correct. I should have been more precise.