If you want to ineffectivly filter out most candidates just auto-reject everything that doesn’t arrive on a timestamp ending in 1.
Really, the better test would be to not discriminate on it before you know it's useful, but store their answer to compare later.
There's a bizarro version of this guy who rejects people who do it in their head because they weren't told to not use an interpreter and he values them using the tools available to solve a problem. In his mind, the = is definitely part of the code, you should have double checked.
That does change it. In that I can see how false negatives may arise. Though, when hiring you generally care a lot more about false positives than negatives.
This isn't a good methodology. To do your validation correctly, you'd want to hire some percentage of candidates who get it wrong and see what happens.
Your way, you're validating whether the test is informative as to passing rate in the next stage of your hiring process, not whether it's informative as to performance on the job.
(Related: the 'stage' model of hiring is a bad idea.)