You're right. His pupils appeared black AND we saw the whites of his eyes. Much different than his usual staring-at-the-sun squint.
Biden was taught to look at Wallace or the camera to retain focus....that’s what you teach to Dementia patients early on.
Perhaps he was wearing “smart” contact lenses & reading a prompter. They are black.
https://techxplore.com/news/2020-01-invisible-startup-unveils-smart-contact.html