In LK, we made a change for PvE reasons to let some of these stuns also cause an interrupt, since many bosses were immune to stuns.

It was never the intent that they cause both a stun AND an interrupt. The original change was made to let the interrupts work on stun-immune targets (generally a boss). A target upon which a stun has DR'd is not the same as a stun immune target. The original change was not intended as a stun buff for PvP.

Bottom line: the interrupt portion of these abilities will never happen against players. (source) -GhostCrawler

Well, let's take a look back, shall we? It seems that I can find a place where the stun WAS intended to be used as an interrupt against players who had become immune through diminishing returns...

Hammer of Justice: This ability will now still interrupt casting even if the victim is immune to the stun due to diminishing returns. (source)

That's right... it's from the 3.1 patchnotes. It's not like I went through and found this statement in season2... this is something that happened a couple months ago deliberately. So, either GC is completely off base on this one, or is just lying for some unknown reason.