10x + 1x + 1x = 12x
Now you convert that 10x engineer into a 10x multiplier, and let's assume he doesn't actually do real work anymore because he's helping everyone else be more productive:
0x + 10x + 10x = 20x
He may no longer be doing any work, but the rest of his team is more productive and significantly outstrips his own individual output.
That's some very impressive made-up math!
Obviously the real world is not so simple, but the effect is the same. There's the engineer that cranks out a crazy amount of code and nobody can keep up, and then there's the engineer that helps everyone else crank out more code - now everyone is learning and increasing their output. Which would you rather have on your team?