it seems the only factor they were checking for (?) was arterial stiffness? if you're arteries aren't stiff but you're hyped and anxious out of this world and have a very elevated heart rate... not sure the stiffness is what matters anymore
1 cup typical drip coffee = ~70-120mg caffeine
25 cups = ~1750-3000mg caffeine
Examine (https://examine.com/nutrition/caffeine-consumption/) recommends up to 400mg/day as safe for healthy adults as in "most healthy people won't experience side effects at this level". It seems like 1000mg is the "beginning" of toxicity for a healthy person. Of course, half-life and time between consumption of different sources makes a difference too.
But 1500, 2000+ in a day? Phew, maybe I'm overly cautious but that just seems unwise for anyone