For my view, consider a very bright problem solver, who bursts into tears every time a person watches or criticizes their code. Do you think that's relevant to performance?
Certainly is a balance, but studies like this usually wilfully ignore all of the other aspects of software work, that aren't sitting in a chair alone writing code.
When being interviewed, an entire career is resting on your 45 min - 3 hr performance. And at certain places, that could potentially mean the difference between $50k/year, and $250k/year.
On the other hand, you're not risking your job every time you discuss something with your co-workers.
There will always be absolute edge cases (like the person you described, which by all means sound like an extreme case) - and I think it's fine that some tests manage to exclude these, but the problem is that it's also potentially removing candidates far away from the edge.