I suspect the reason for not saying it in those terms is the enormous storm you'd have seen on the internet claiming that Microsoft were trying to kill off Flash by bundling Silverlight into Windows. From a political standpoint, a few slightly confused or irritated Silverlight developers is probably a lesser evil.
AndyC, the only hole in your theory is the fact that Silverlight didn't turn out to be the threat is was thought to be. In 2007-2010 (until IE9), i was really scared that Silverlight is going to take over the web.
More generally, I think the association of Silverlight and Flash(for example saying Flash/Silverlight whenever talking about plugins) is something created in the marketing department at Microsoft and quickly adopted by Sliverlight developers. My feeling is that it has no base in real life where, i believe, Flash is one or more orders of magnitude more prevalent then Silverlight.
I hate both, and wish they weren't on the web, but to Flash' credit is the fact that it was created around 1996, while Silverlight was created in 2007! That is why Microsoft is irrelevant in this webby world, and NOT A THREAT TO ANYONE ANY MORE. The stock market seems to understand this.
[I'm talking only about the abomination that is Silverlight-for-the-web, not dotnet in general or Silverlight for your intranet]