If the ad suggests it can start a car, I call b#llsh#t ! That device will not be capable to jump start a car in the same way as the you can use another car or a proper jump starter like this:
This one is much larger because it contains a lead-acid battery
Notice how it uses big-#ss cables for huge currents
When charged this thing can start a car with a fully depleted battery immediately or at least within minutes.
That MiniMax is a completely different product, it's a powerbank with a 12V output option. Maybe if a car battery misses just that little bit of power it needs to start the car, a MiniMax might crank up the voltage just a bit (you might have to wait half an hour for the car battery to be charged by the MiniMax) so that you can start your car.
But looking at the shape, the MiniMax will contain a few 18650 cells. But these will by far not contain the same amount of energy as the device in the picture.
And regarding buying 18605 cells or devices containing these: you get what you pay for. If you buy cheap ones I guarantee you that these will be a dissapointment. Good 18650 cells are made by Sony, LG, Sanyo, Samsung for example. Their capacity will be 3500 mAh maximum each. Cells with a higher capacity are "stretching the truth". These cells from reliable manufacturers cost about 4 to 5 US dollar each. At a lower price I would not trust them to be any good and not worth buying.