“The king has said he eats a heavily plant-focused diet...”
Yeah. That will cut you short!
His “exercise plan” consists of Stretching, very mild sit-ups, back extensions, pushup from the knees and running in place. He’s finished in 11 minutes....
The link you post has a slightly misleading title—although no fault of yours. He is the oldest person at the time of his coronation.
Besides QEII and Victoria (who were Queens), George II and George III both were still reigning at an older age-—and Edward VII died at an older age, but after resigning at a younger one.