a sodding big capacitor is a stylized way of saying a large value capacitor, e.g. 20,000uF,
the fact that its going through a switchmode converter is probably your first clue of where the limit comes from, most modern ones in this type environment would have some form of current limiting, thus when your hard drive tries to turn over and would happily draw 17A for a few millisecond if it could, it hits the limit and voltage goes down, and they likely sized the output capacitance of the hub to be just big enough to cover one drives demand,
now to make clear you should be able to connect more than one external at a time, heck i can do that on my unpowered hub, but you have to start them one after another, or the demand of all the devices at once is just to much and the brownout circuit kicks in, as the largest power requirement of a drive is getting it spinning,
final point for the night, have you actually tried connecting as many drives / devices as possible to the thing and seen if it works? or are you fussing over a cheap product only claiming what they can promise to be true in all situations, I've done things with USB that break the specs before, heck I've had a webcam running at the end of a 60m usb extension lead, until you try it and can definitively say this doesn't work, why make a fuss?