There is no mention that once validated, that the RAPPOR-based metrics when fully deployed would be take from a random population. Only that the initial study of the system will be done to a random population.

FTA:

What we plan to do now is run an opt-out SHIELD study [6] to validate our implementation of RAPPOR. This study will collect the value for users’ home page (eTLD+1) for a randomly selected group of our release population We are hoping to launch this in mid-September."

Notably:

"this study will collect ... for a randomly selected group"

[6] - https://wiki.mozilla.org/Firefox/Shield/Shield_Studies

I added the second part about the random sample later to the comment when I moved the proposal already out of my short term memory. I hope they use the data from their initial study to test whether the opt-out group actually is different from the group they already get data from.