Think about it for a minute. There is a LONG cable from the battery to the jump points. It is a large enough gauge to support the minimum current needed to boot up the hybrid's computers and allow it to start. It likely is NOT heavy enough to support the hundred amps or so that is needed to run the conventional starter in a different vehicle. In this case, the direction of the current does make a difference.
Mind you, the cost of incorrect polarity rises to a Max of $8000. (some inverter units have field serviceable DC to DC converters, for 'only' $600 each)
The point of the logic is that with the Prius as the donor vehicle and a non-hybrid as the recipient vehicle, the current flow may exceed 100A in adverse conditions. That current flow would zap the 100A DC/DC fusible link if the Prius was READY; or possibly the 120A MAIN fuse in the battery positive terminal block. If you hook the jump cabling directly to the Prius 12V battery then no Prius fuses are in the picture if the Prius is IG-OFF. It would be better not to use the Prius as the donor vehicle to avoid these issues as well as the potential issue of zapping the inverter in the event of botched battery polarity.
2004-2009 Toyota Prius Inverter Assembly G9200-47121 | Boch Toyota South $2000 to $3000, not installed. A jump start pack is $30.
Another car, other than a Prius, needs considerable current to be jumped, hence the typical robust jumper cables. It may be that the path from the 12V battery in the rear to the exclusive jump starting terminal in the fuse box is not sufficient to carry the current draw to the vehicle that needs to be jumped. Direct from the 12V battery through large conductor jumper cables is probably a path of much less resistance.
Can a Prius jump start another Prius? Well I think there is hidden camera footage of it happening in the wild. But I don't think it's ever happened in captivity.
I used a jumper pack last night. 12 Volt went from "No worries!" at my house to flat dead five minutes later. My jumper pack got ME home. All I had to do was climb through the back to access the pack which was stored back there) and the 12 volt battery. I just left the little cover over the hatch latch off. Fact is, I tossed it.