Not sure why they conclude that "bergamot essence may induce muscle cramps" etc based on a single case. What if the man had an allergic response of some kind and 99.99% of the population would be fine drinking so much earl grey tea?
Really, they are just reporting the surprising fact that it is possible to overdose on bergamot by drinking a large, but not unreasonably so (at least by UK standards), quantity of tea.
This of course is only one case report, so one should not assume that one needs to drink 4L of Earl Grey to induce these symptoms. I typically drink about 2L of tea per day, yet 2-3 cups of Earl Grey makes me feel nauseated. I've never had cramps or vision problems, but that's probably because I stop when I start feeling ill. This is good to know for future reference.