In the linked thread a couple of people offer some theories. Here's the TLDR list:

1) Possible that WMP loads DirectX sound/video drivers so WoW doesn't have to.
2) Someone supposedly knows a Microsoft Engineer who says something about MMCSS priority and how WMP is the only program with it. It forces a certain timing scenario to occur with processor instructions and causes a marked improvement in WoW.
3) WoW's new multi-threading support in 2.3 makes WoW time itself differently.

Personally, 2 sounds like it might be plausible, but that does not explain why some people get similar results running Quicktime instead of WMP.

I'd lean towards 3.

It should be noted here that not every user sees the improvement.