No, it doesn't mean that.
You self-select only in the sense that you choose to register with them in the first place (they say they advertise for members in a number of ways with as broad a reach as possible)
At registration, they collect demographic information about you
Then when there is a poll, they select a representative sample, and that is who receives that poll (not everyone receives all of them). Then the final data are statistically weighted to reflect the adult population (so that those who don't return their answers don't skew the results)