They're only similar in that they both use markup and code behind. But that's like saying Silverlight and classic ASP are almost the same. With that said, I should have said, at introduction Silverlight looked fundamentally different. Flex has copied what I think is Silverlight (and WPFs) main differentiator, which is lookless controls (which ties into binding).

I think you underestimate the amount of people who have used Flex at some point in time.