Eustace Osgrey is an bitter old man who is very much delusional when it comes to remembering the Blackfyre Rebellion.
You're right of course that he is a bitter old man; though I wouldn't call him delusional, rather wishing the outcome had been different - which it might well have been, had not Baelor and Maekar crushed Daemon Blackfyre's rear, giving Brynden Rivers and his bowmen the time necessary to take the Weeping Ridge.