I think they'll get in due to the fact they won a slam though I may be wrong.
I could be wrong but is there not a rule that of you win a slam but are outside the top 8 then as long as you finish in the top 20 you qualify for the World Tour Finals I am pretty sure that's how Goran made it in 2001!
That sounds plausible. I guess it's just so rare that a Slam winner doesn't make the year end top 8 that it rarely comes up.
As said above though, they will barely have played together (not that it mattered this tournament!) Hopefully they can play a few Masters together (where they would be seeded) when Nielsen isn't playing Singles.