Well I cant for certain tell how many stuns were interrupts, but finishing garrosh normal with 3-4 interrupts (plus stuns/fears myself via shadowfury/howl of terror) and the next interrupter is at 1 or 2 gives me a pretty good idea. Also, you rarely need to interrupt in the first place during phase 2 as they should be damaged enough to break prior and if its a healer who was MC'd and you stun them good chance they could die due to debuffs ticking, especially if using the stack method (which I personally dislike, its not hard to move and doesnt really improve dps much but can easily wipe the group or stress healers around iron star impact damage).
And yes thats kind of also the point I was making, the rest of those PuG's generally suck and fail at many things they shouldnt, and normally the worst of them are people sub 555 ilvl, although I care more about how they are gemmed/reforged than the actual item level. Normally you get people who do what they are meant to more often at the higher areas. Then again, no reason a lock cant interrupt as well as any other class on that fight, you cant get more than one MC interrupted per cast of touch of y'sharrj, so the fact that a warlocks CD is 24 seconds not 15 like some is irrelevant. Interestingly though, quite a few locks ive seen dont even know they have an interrupt, and some of them have been doing SoO heroic bosses. The people I prefer to avoid are those which dont know what a personal self CD is, or what a raid CD is, or even methods they can use to reduce raid damage taken (aoe stuns on adds, anti magic shield, twilight ward etc).
People saying everyone should be higher than 540 etc, quite a few of those are alts you need to remember. I have 5 toons over 540 ilvl, out of 13 level 90's. Im still competitive for my ilvl (as in I pull about what I should at say 520 etc) on those other toons but I dont have time to LFR that much per week. That and ive found alot of really good players in those lower ilvl areas, its just less the normal now and far more common to find people who have only done LFR previously.