"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."
They're 20 ounces so nearly twice the size of an average drink. Even energy drinks which are typically bigger are usually 16 ounces (though sometimes 20). Looking at the website the highest-caffeine version has just under 160mg in 20oz.
Regardless, not sure what this has to do with the overall point, which is that you could consume multiple "high-caffeine" drinks and still be well under the amounts discussed in this study.
I think you are using energy drinks in your example for effect, only because people overrate their caffeine content relative to coffee. Coffee drinkers who have a full tumbler/togo cup in the morning and a sbux or continual refills from the office pot at work can far exceed the caffeine intake of 3 energy drinks. Meanwhile, Red Bulls actually have deceptively little caffeine and are barely worse than just a regular Coca Cola.
It’s very easy to get into the habit of extremely high caffeine intake, speaking from experience. Not only is caffeine in many drinks and extremely well distributed, its psychological effects are pretty mild. But I think the worst part is that people don’t really think about how much caffeine is in the coffee they’re drinking because it’s hard to measure.
[0] https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-h...