He also says it's a 13 issue and I don't see Xal making it to 13.
The Dark Heart makes it to Midnight at most. To me, the whole point seems to be to have tons of Shadow/Void energy to infuse the World Soul and turn it into a void entity (Void Titan, Void Lord, Void God, whatever). This will be thwarted by us at the end of TWW causing Xal to pivot into the assault on the Sunwell. The Dark Heart might still play a role there, but it certainly won't after we "banish the shadow forever".