I can see being hesitant to connect the jumper to the starter directly without the truck battery as an intermediary. That could mean higher currents than with the truck battery there and maybe damage to the jump pack. And if you use the regular battery cables without the battery, there may be odd things going on between the alternator and the jump battery after the engine starts. Without knowing the specs/inner workings of the jump pack it's hard to say. I would try to ask the manufacturer first.
That out of the way, I would think it's probably OK for a test to connect the jump pack directly to the starter and see if it can turn the engine, keeping the cranking very short. That could damage the jump pack of course, but then again, if the jump pack can't actually turn the engine, it's useless, and you could be in a predicament out in the boonies. And those things aren't that expensive.