The way I see it, the main contributes to a lot of the heeling forces when sailing downwind with spi. Easing the main will remove drag and heeling forces. You depower the boat, not just the spi. The main also create some drive, so there is a balance.
If you sheet in and oversheet by doing so, you are creating even more drag which only helps in slowing you down and pulling you over.
Sheeting out on the spi is of course first priority. I think you need to put the whole thread into context to make sense of it.

Just my take on the topic.