When I first started scrolling I got a little confused b/c I didn't realize the header was fixed. The sign up button disappeared and I couldn't figure out how to get it back.
Yeah I find it confusing too. It didn't feel like I was scrolling down, just covering the sign in page. It's weird and non-simple.
I'm not sold on the idea of having all content combined into a single page. It's awkward to navigate, but an interesting choice nonetheless.