No, they are not. Because they are two whole different things.
Vocal Minority and Silent Majority is the affirmation that the vast majority of satisfied customers will not go out of their way to praise the goods they are enjoying, while the unsatisfied consumers will almost always express their feelings. Sometimes to the provider, sometimes as negative advertisement.
Sample sizes is a statistical data point. The larger your sample size, the more accurate your data reflects reality. However, above a certain number, you get a close enough image to reality that increasing the sample size will not affect your outcomes that much. So, you need to understand the scope of what you are studying, and then do your best to not bias the sample pool.
We all know since the start that Overbuff (and other similar websites) have a datapool biased towards the higher ranks, because the data collected need to be voluntary, and mid to low rank players don’t know or don’t care about inputing their data into a 3rd party website.
We also know that private profiles made Overbuff sample size smaller.
We do not know if the private profiles pertain to a specific group of player or now. So unless Blizzard or even Overbuff itself send us information about that, the reasonable thing to do is assume the private profiles are on the same distribution as the whole datapool.
So the only question is if Overbuff datapool is still big enough to be considered relevant. I’m inclined to believe that it does, mostly because that the only datapool we have. But if you believe that it’s datapool is not big enough, then just disregard everything involving it, because the well has been poisoned a year ago, and it will not get better, unless Blizzard finally release a public API for Overwatch.
(Which is a good request, and I think more people should support it)