Technically, the {{ }} is just part of an id of an element.
Personally, If I am loading templates from script tags (which I rarely do), I'll use the staches to differentiate the template id from other string literals in my code. It also let's me be absolutely sure that If a template's script tag is injected into a page, I don't have a name conflict.
That's exactly what that last piece of code means. This is likely in the defaults of the Control and the dev expects that in the usual case a script tag with an id of {{someName}} will be used, but another script id could be passed in or a URL.