Earlier quoted context omitted.
Async JS doesn't block either - and cross-domain scripting rules are strict enough that it is pretty safe. The iframe code has an async Google Analytics script in it - perhaps that is the reason for the iframe (to use google analytics for tracking usage).
Curious what you mean about cross-domain scripting rules and safety. Normally, loading a third-party script via a script tag lets that script do anything the page can do, which makes it decidedly unsafe. Personally, I greatly prefer the iframe.
iFrames do offer a little more security here - but in this case I would still go for just script since I don't see that the downsides outweigh the added security. (you can still do a whole ton of damage from an iframe)