لَكِنَّهُمُ اخْتَلَفُوا فِي أَنَّهُ هَلْ يَجُوزُ عَقْلًا أَمْ لَا؟
They differed over whether this is rationally possible or not.
Revelation determines that a person dying in shirk is not forgiven. A separate inquiry asks whether forgiveness would have been intrinsically impossible on rational grounds. One position holds that it is rationally conceivable but excluded by revelation; another argues that divine wisdom itself precludes it.
Possibility is not a report of actual occurrence. Confusing those two changes the question being debated.