"...black-shirt there said...."
I stopped right after that. Honestly really? You're gonna listen to a black shirt? Here's a true story for ya, 2 months before finecast came out and the rumormongers where talking bout the "shift" to resin i went up to my local black shirt and said "Oh did you hear the rumors bout the shift to resin? He stated "GW isn't doing resin i was just at GW headquarters and they have nothing in resin nor are they going to discontinue making in metal." He was wrong. Then he was fired for some random thing.
Before that when the rumors of CSM where floating around and people were crying out "brokenz!! sky falling if controlling enemies movement is true!! (lash whip)" (yes there was a time when people cried out that CSM was broken har har) Local black shirt stated "There is no such thing, GW would never allow such an ability. It's too stupid and would upset the local tournament scene." He was mildly correct, well more towards the latter, as lash whipping DP's forced people into Rhinos more which shifted everyone into more tank battles. But he was wrong on the first part.
Don't even get me started on what they said about 'nids.
TL;DR?
Black shirts know as much as us, even then i personally believe they know less, based on my experience of course.