That's... not entirely accurate. What do you think is rendering all those gifs to screen?
Just did it on my 2011 MacBook Air. Pushes all four cores up... about 15%.
15% of 4 cores ~ 60% single core capacity. That's distinct from saturating a core both qualitatively and quantitatively, which means the prediction is not a bullseye... but it's on the order of magnitude in impact, which means they're not entirely wrong.
Two cores went up 20%. The other two didn't budge. Go figure.
edit: such as https://giphy.com/sports/2019-stanley-cup-playoffs which seems to be entirely gifs if you view page resources in the debugger.