This could have been an oversight since Lewis did write MN later than most of the other books and he might not have thought of this, but the situation is also a bit different in this case.

My guess is Aslan told Digory what would have happened in this case because he was trying to reassure Digory that he did the right thing. This was before Aslan didn’t want Digory doubting his decision.

I can’t recall many of the times when Aslan didn’t tell people what would have happened, but one that does come to mind is in VDT when Lucy uses the magician’s book to spy on her friend. She asks Aslan if she has ruined the friendship by her actions. He reminds her that no one is ever told what would have happened. I think he doesn’t tell her what would have happened because he doesn’t want her to dwell on her mistake. He wants her to move forward and focus on forgiving her friend. There is no reason for her to dwell on something she can’t change.