As you've read the description I'm sure you noticed its done with magnets. And you've read, "…triggers in the glove of the weapon hand that cause the magnet to reverse polarity when brought near it." I didn't notice anything about magical invisible arms which can move the weapon anywhere you want. The weapon only propels about half a foot or so straight away from its mounting. If your hand isn't there to catch it the weapon just hits the dirt, and would look like a real-robot anime mech ejecting a weapon that ran out of ammo.
I'm sure you also read, "allowing him to go from unarmed to deflecting an enemy blade with his own in the blink of an eye." Emphasis mine. I didn't know unarmed now means "having a weapon in his hand". When did that change happen?
Common sense (which the rulebook states GMs should use, things which are impossible are impossible) indicates the di-pole maglock does not magically teleport weapons to your hand wherever your hand may be. The actual rules also indicate you must be UNARMED to use the quick draw talent to parry. The rules also still say free actions may only be used durring your turn. Only reactions may be used when it is not your turn, a reaction does not include a free action.
The description also states that "versions using verbal and more sophisticated somatic cues also exist" and does not list a distance that it propels the weapon.
The "allowing him to go from unarmed to deflecting an enemy blade with his own in the blink of an eye" sounds more like flavor text than them saying this is the only way this item can be used.
I finally see the difference in our views though. You assume the melee weapon is on the marines hip and I assume it is on his chest (because that is where I always have it when I use a dipole mag-lock). Based off of your assumptions, my understanding of the dipole mag-lock doesnt make much sense but in mine it does.