You played piano for 8 hours straight, but were you 1) playing well, 2) repeating the same things over and over again, 3) doing something creative, or 4) any or all of the above?

There is definitely a potential that the study design offered a measurement that wasn’t useful in determining how well these drugs do or do not work.

But, in your example, you measured the length of time you put into playing the piano, something the study did address and note that people on the drugs did do—by increasing their time on task. The important part though was that while taking longer, the participants did not perform any better when on the actual drugs and worse than those on the placebos.

So, following this logic in your own claims, it’s possible that while you played for 8 hours, it wasn’t your best work.

I knew someone who took something and played minesweeper for 8 hours straight. I'd say both are equally productive.

This sounds like what Ritalin does to hyperfocus. I personally think Ritalin makes hyperfocus much worse than when not on Ritalin - from my own experience of the drug for the treatment of ADHD

I've seen more than a few people on various drugs do some obsessive things, continually for long periods without a break. That didn't make them smart or skilled.