It is the combination of Caffeine and L-Theanine. Drugs/compounds in combination can frequently have different effects than just smashing the lists of effects of the two together. Further, the effects of stimulants are not simply "increases anxiety, heart rate, reduces concentration, etc". Look at cases of ADD where stimulants increase concentration. Look at studies where low doses of caffeine increase alertness without the "bad" stimulant effects.
As for your strawman about yogic monks: the article did not say "all monks..", just "monks...". The difference here is that without the qualifier, english assumes the statement refers to a significant portion, but not all or even most. Further, some traditions, such as various forms of Buddhism, particularly those that practice sitting meditation, do in fact have tea as part of the meditative ritual.
tl;dr - you are way over-simplifying and being generally disingenuous
A lot of the cases with ADHD are very specific to the type of ADHD. Usually things that somehow alter norepinephrine or Dopamine levels in the brain (as Caffeine does) often alter somebody's response with ADHD.
For me, it doesn't work. I'm just too wired, my heart feels like it's going to pop out of my chest if I take an amount that will affect my actual concentration (not the same as alertness). Adderall, to me, feels like a much smoother caffeine, and I can concentrate. However, I hate the way it also makes me feel in high doses. Atomoxetine worked great for me as well, but had weird "sexual" side effects. Right now I'm mostly on Wellbutrin XL, which is a norepinephrine reuptake inhibitor, and while it doesn't work as well stimulants, it does work to a point and I feel like my normal self and even better. So far, the best combination for me has been Wellbutrin with a small kicker (5-10mg) of Adderall. Before Wellbutrin, I would take 20-60mg of Adderall a day. Also, Wellbutrin helps with some of my natural anxiety as well.
Also, for what it's worth, Adderall (a stimulant) tends to decrease anxiety for me in certain situations, while amplifying it in others. It's a weird relationship. In some social situations where tons of things are happening, without Adderall I can't focus on anything and I start to go on a weird panic mode. With adderall, that decreases because I can ignore certain things. But it also decreases with alcohol too, because I just don't care.
Anyways, the best thing I ever did was see a psychiatrist. Treatment of ADHD sucks because it's hard to know what will and what won't work, and what side effects are involved.
The idea that stimulants only increase concentration for folks who suffer from ADD is bogus. I don't have the research at hand, but studies show that this effect is not unique to sufferers of ADD.
As for your strawman about yogic monks: the article did not say "all monks..", just "monks...". The difference here is that without the qualifier, english assumes the statement refers to a significant portion, but not all or even most. Further, some traditions, such as various forms of Buddhism, particularly those that practice sitting meditation, do in fact have tea as part of the meditative ritual.
tl;dr - you are way over-simplifying and being generally disingenuous