The British precautionary measure was to only give it to pilots? I don't know how meth works, but as with most substances that affect behavior and judgment, it would seem a pilot would be the last person I'd give something like that to.
As someone who is essentially taking amphetamines most days now. At controlled doses, they don't really affect judgement at all. In fact, they can measurably improve cognitive performance. For me as someone with ADHD, I don't really feel anything at all, it's just replacing the neurotransmitters I don't get enough of naturally so the only effect is that I'm more emotionally stable and have more motivation and concentration. For someone without ADHD the effects would be a bit different, but neurotypical people take ADHD meds (which are essentially low doses of slow-acting amphetamines) in order to study all the time because they do improve cognitive performance for a lot of people.
I think the whole "meth is not a stimulant" thing probably comes from the fact that stimulants don't always make you want to run through walls, in fact stimulants can make you feel more relaxed and calm. It kind of depends what exactly they're doing to your nervous system and what the dosage is. When people use stimulants recreationally they typically
want to feel high and euphoric all the time, but that's generally not the case when you're using them in other contexts.
Using meth on soldiers was probably a bad idea though.
Drugs which work like amphetamines are also used as diet pills. In fact, amphetamines used to be very common in diet pills before they became controlled substances. It's one reason why women in the 1950s were so skinny.