Not sure how multiple exposures helps?

Smaller sensor, tighter aperture. So yes, more light or a more sensitive sensor.

They must mean by creating a composite image with multiple in focus areas? Otherwise I agree, I can't see anyway that multiple exposures would help, at least from some light reading on Wikipedia - https://en.wikipedia.org/wiki/Multiple_exposure

https://en.wikipedia.org/wiki/Focus_stacking :)