I think it is more that this particular tab was somehow responsible for eating half of his battery life. It's not particularly clear that that particular page would consume an an insane amount of CPU/Battery.
My point was that you cannot simply see the things clearly from the Activity Monitor. And many people are not aware that the Task manager exists in Chrome.
Again, all true, but I don't understand why it's here.
But if the developer of that site left somewhere in JS something like while(true) {...} that page will drain your battery, even if you're not looking at that specific tab. And even if you are in other app than Chrome.
I like to play FTL, and it makes my laptop sound like a vacuum cleaner, with a 1.5 hr battery life. And that's a freaking turn-based roguelike. But I can run an old SNES emulator for an hour and barely touch my battery.
Go figure.