That's what people always say. Like, as if they actually tell from 1 or 2 of these goofy whiteboarding sessions.
It seems the most they can actually glean from these sessions is, "Huh, looks like their problem-solving approach is different from mine. Plus, they come in not having the advantage of having the solution already ready-baked and laid-out for them, in whatever interviewing guide it was I found this problem. Doesn't that suck for them? Not to mention being under pressure from doing way too many, generally carelessly orchestrated interview tracks (in their non-existent spare time) while being stuck in some job they're anxious to quit. And getting distracted by me diddling with my cell phone while they're doing headstands for me. Gosh, why is it so hard to find decent candidates, these days?"