Now one of three things is happening here. Either Mát is misremembering (it happens to all of us), he or she is lying, or he or she has made the mistake Daxxari mentions, mistaking the fact that Resilience back then was part of the item budget and thus, PvP gear was less powerful in PvE because it spent itemization points on a stat that reduced your chance to be crit (back then, that's what Resilience did). But no matter how you look at it, the idea of this penalty introduced for wearing PvP gear in PvE did not exist - which is why so many of us wore PvP gear to PvE in. Sure, it had resil on it, but it was easy to get and often better than what we would have gotten from five mans to prepare for raiding Karazhan.
With a game as old as World of Warcraft (we're entering its tenth year) this is understandable. Not all that many people playing today have played since launch, not even since the days of BC or Wrath - heck, there are a great many people who started playing in Cataclysm and even quite a few who started during this expansion. People will tell you that the talent system that we had up until Cataclysm allowed for great customization. They may even believe it.