Since gnash can play youtube video, this isn't a restriction in practice. Much ado about very little.
You may be correct that Gnash can currently play youtube videos. But you're missing the point of this article. FTA:
Citizens should be able to download transition-related content in a way that makes it simple to share, excerpt, remix, or redistribute.
The issue is not only being able to view the content using free/open software but more generally being able to do something useful with the video without having to jump through hoops. Especially hoops that have difficult to decipher legal encumbrance.
Today's scientific question is: What in the world is electricity? And where does it go after it leaves the toaster? -- Dave Barry, "What is Electricity?"