Back to the topic of the 'gray zone'. There is nothing in this blog that says that umpires are not penalized (ie are given leeway) for incorrect calls made in a gray zone. The study in fact states that most of the errors are borderline, and that the decline in error rate as you move away from the plate is "rapid".
That said, I have seen some sites, not all, that will measure umpire accuracy according to what they term the 'called zone' rather than the true zone. In other words, a zone that has become 'accepted' by umpires, batters, and MLB. Is it okay that the interpretation of the zone has changed from the defined strike zone? Probably not, but MLB could fix that by easily enough by enforcing the actual strike zone.