On that private server mages using this glyph were still able to teleport when stunned, but after the use of it the stun still remained. A bit more simply: I think that this glyph could be used when you are stunned, but doesn´t break stuns. Am I wrong?
You should not be able to Blink while stunned at all if you glyph Rapid Displacement. It's a PvE glyph if you have a spare slot and want extra mobility on encounters.
Are you sure, or you just guess? I want an answer from someone who played on retail.
I have never played any MoP private realm, including this one, only retail. As of March 2014 (when I unsubbed) glyphed Rapid Displacement did not let you Blink while stunned and was a niche PvE glyph.
EDIT:
Forum threads from the time it was changed indicate that for a short time, if you glyphed Momentum and Rapid Displacement, you got two stun-breaking Blinks. This was hotfixed and should not be reproduced.