Having visited Seattle in the past, I can attest to the topography not being very conducive to using a translator to make an "analog station" out of a HD subchannel.
Having said that, I agree with your alluding to those two stations having a target audience which is much more likely to own HD-capable radios, and that can definitely have an impact on ratings if even a handful of those in the "rich clientele" carry PPMs.
It's obvious that there are exceptions for situations like these, but the fact remains that in the majority of markets where it is technically feasible, HD subchannels are used to feed translators, and the Nielsen policies do not allow us to come to any definitive conclusions as to what percentage of such stations' listening come from which transmitter (we have a similar such dilemma on translators combined with parent AM stations).
I think our best guesses are that in cases where all of the circumstances favor either AM with a translator or HD with an analog translator, most of the listening is to the translator. But what the percentages are in either scenario is anyone's guess.