His mind for organic chemistry might still be sharp as a tack. But at 84 years old, if he was the students' teacher during the initial part of the pandemic there's probably a chance he struggled with the digital adjustments to doing so many things online and delivered a subpar overall experience. If nothing else, perhaps eyesight and hearing fades a bit with age and makes trying to do stuff through Zoom and other online tools something very difficult for them making office hours and other things a challenge everybody resisted. This means that these students were far behind other years students.
By the time they all resumed in-person activities, the teacher might have thought that his students were at the normal level he was used to through decades of experience, but he misread how far behind they were.