Personally I think it's pretty noticeable. My eye went right to it the first time I saw it.

I suppose you could argue that they should have an overlay spinner (or something similar) across the entire page so it would be impossible to miss, but that's not as elegant.

Also, why wouldn't your reasoning apply to the url progress bar in many browsers that you see for an ordinary http page load? It's up their at the top of the page, why not put a nice unmissable transparent loading bar across the middle of the page?