I give even less credit to the media's understanding here. I suspect that they weren't intentionally draining the battery just to satisfy a test condition and instead were running through random permutations of user actions or something and making sure it didn't result in bugs/slow performance.
I suspect that the engineer's complaint was that running these tests would have the side effect of using up the phone's battery on things that the user did not initiate or knowingly consent to.
If Facebook wanted to test things on phones with low batteries, rather than intentionally drain the batteries first, I suspect that they'd just target phones that happened to have low batteries at the time. That could be troubling, but I'll reserve judgment until we have concrete evidence of that occurring.
You'd think that given their large userbase, they could design tests so that they have miniscule impact on individual users. It's still possible that this is a massive issue worthy of scandal, but until we know more information, I'll take secondhand information from a single disgruntled employee with a grain of salt.