It's actually amazing how he can still pull a song together.
I read an interview with Lady Gaga where she talked about how hard it was to watch what the Alzheimer's had done to Tony. And yet with all that she said ad soon as the piano hit the fist note he snapped into performance mode and was spot on. When the song was over he may not have known where he was, but for those 43 or 4 minutes the was his old self again.
It really is amazing how the brain works (and sometimes doesn't.)
They did a great job on that album. Considering that it will very likely be Tony's last, it is great to see him go out on top.