Yeah, I'm not so sure about this connection either. I played soccer at a pretty high level throughout my teens and still kick a ball around and have good skills but my boxing footwork has been a long time in improving and I would still only describe it as average.

Part of the problem is fitness - it's bloody hard to have good footwork when your legs feel like lead.

That said, my footwork is starting to come on and I attribute it to four things:-

1) Heaps of shadow boxing at home, focusing very intensely on footwork - starting slowly and gradually building the combos and slips up to speed.
2) More skipping and varying the way I'm jumping.
3) Shoulder sparring (a method of training that our trainer uses where the goal is to touch the other persons shoulder with your glove - the aim is speed and getting in and out fast on your feet. Sorry if this is a common thing, I am unaware)
4) More sparring and ring work.