Each case is decided on its own merits. If Google fired union organizers - which, since they settled, is not even a legally established fact (that's likely why they settled, to avoid having any facts to become legally established) - that doesn't mean they fired this particular person because of it. There are thousands of people in that union, which weren't fired - and that's what the opposing attorney would point out the first thing. And you'd need to establish some causal link between union membership and firing - with actually legally acceptable proof. Getting such proof is what I think of as "low chance". Separately, I also don't think joining a really toothless union would be the cause for firing - unlike the other things described in the article, which most likely were the real cause.