Microsoft has nothing to lose in this, every mention of this story in the press helps them. They are probably hoping for a lawsuit
Also, if WP allegedly has such low adoption that Google doesnt feel the need to make an app, how much revenue can the WP app hurt?
Vimeo and Yahoo mail have WP apps but Youtube and Gmail don't. Go figure.
The download button, on the other hand, is a completely different kettle of fish. If people start using Microsoft's Windows Phone YouTube app to download and distribute YouTube video I think that will have tons of impact, way beyond the existing, lower-profile downloaders, no matter how tiny WP's adoption is.