That's quite a few pieces for every man, woman, and child on the planet.

I wonder how many cubic kilometers of plastic that is

Significantly less than one; probably just roughly a cubic hectometer at most. A cubic kilometer corresponds to about 1.6 x 10^15 1x1 Lego blocks, so over a quadrillion. 600 billion is only 0.6 x 10^12.

400 Lego 2x4s makes a kilogram. So let’s say a cool billion kilograms.

And since Lego essentially never is thrown away (except by spite or accident), that's more or less 1 megaton of carbon capture right there.

I don't think you get to count plastic you produce from extracted sources as carbon capture.

As long as you don't burn it, it's pre-emptive CCS. At least that's what I tell the investors.