Well, you can't. In this case I believe they're already pretty confident about who the PIE speaking people are (the "Yanmaya") and this study is about tracking down where they originally lived. And they have shown that they mostly replaced the previous European population rather than transferring the languages to them.
David Reich is aggressive about these genetics results though. IIRC I read a NYT story once where he came in and claimed to have upended all of Polynesian history based on the genetics of a few historical skulls they found, but it didn't seem like strong enough evidence to me.
Maybe this is how the branches of Indo-European evolved.
Laryngeals replaced by vowel lengthenings, merging of consonsants, vowel shifting based on other sounds, etc. It's like there were many different events where "Indo-European with a heavy foreign accent" suddenly emerged.
I can't find it now, but I've seen at least one claim that the ancestral-human-DNA world inside biology is fairly dominated by a clique, and if you're not seen as fully on the team you can't expect to be funded, published and so on. Which isn't to say that any specific claim is wrong, of course, and on the whole it seems very unlikely that they're far wrong on the bare facts, as opposed to more speculative interpretations.
David Reich is aggressive about these genetics results though. IIRC I read a NYT story once where he came in and claimed to have upended all of Polynesian history based on the genetics of a few historical skulls they found, but it didn't seem like strong enough evidence to me.