No one knows, but I still think my theories hold up pretty well. They aren't necessarily trying to find the "best" testers in the sense of who has experience with developing a product or even just in typing. In this, at least some randomness, within reason, can be a plus. It's a plus to have special training in some ways. And it is also a plus to just be a hunt and peck typist. Both will provide WT with different experiences.
So, most people would probably qualify, at least if they have iOS access since there still isn't an app for android, windows, Mac or Linux.
I believe the first small number from the forums were fairly carefully chosen, mostly based on what they posted. Or, more accurately, HOW they posted. I could certainly read posts and make a list of who I thought would give me the best chance of having someone who would be fair. Especially based on posts BEFORE Treg was announced. We had people who criticized WT from the start of Treg, but they gave rational criticisms which, even if wrong, still showed the person was thoughtful.
But other than for a small number, I would have a hard time putting them in a priority order. It would be more like, "These look really good and this much larger group are all acceptable". WT sure didn't make the same list I would have since in at least one or two cases they left out people I was sure would get in - fortunately they got in later.
When it is time to add someone and you are choosing from that second group, it may be just a matter of timing or luck. That is, they may just pick one randomly. Or maybe one posted something recently that they noticed and remember so they get picked.
Now, I could be totally wrong about all this. I just think it fits most of the pattern we've seen. There are a couple outliers I don't even have a serious theory for, but they came in much later.
Oh, and nothing I said should be treated as saying folks chosen later (or not chosen at all) would not be as good as others chosen earlier. The fact is, there is no good way to be sure. So you go with what little info you have. Besides, I don't think it matters if you get the best for this purpose. It only matters that you minimize the chance of getting bad ones. I have no idea if any were chosen who then decided never to report anything to WT. None of us see that. We only know if someone posts about things here or on Slack (WT is not in the slack group).