Shouldn't that be helper_js_global? I thought that was the best place for XF 2 to add external JS.
Put the "<script async src="//pagead2.googlesyndication.com/pagead/js/adsbygoogle.js"></script>" part below "<!--XF:JS-->"
Anyway it won't solve the problem for JoyFreak, you can load the javascript anywhere.
Do you have a link to any articles confirming this? As far as I know ads.txt is absolutely not a requirement, they would sure have notified you in the admin panel if something was missing. I'm not using it as it can only break things for now.
@JoyFreak, if you are using the default ad code and nothing is displayed, you should probably contact Google Adsense and ask for help. Looks like your ad code is fine but Google isn't filling the spaces. They might be blocking it some how.