The only times I've seen items like the Porta-Jump, they're intended to charge the battery, not provide starter current. That said, the key has to be "on" to connect the 5 Amp labeled power port to the electrical system and be used long enough to raise the car battery to where it can start. I'd honestly be surprised if the Porta-Jump worked.
But it's nice to feel more confident having that under the seat! (whether it works or not)