"High amounts" is a bit of an understatement, the study is talking about more than 6 cups of coffee daily, which is going to be in the 550-600mg range. Up to about 400mg a day is safe for adults[0]. 600mg is a
lot of caffeine. For context, a Bang energy drink which is on the upper end of what you can get in a single drink, is 300mg. Most "energy drinks" are around 200mg less. I'm not sure anyone is surprised by the statement "habitually drinking 3+ energy drinks a day, every day, for an extended period of time, might be bad for you."
[0] https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-h...