Builder can be a bit of a pain if you want to do things that are...let us say "questionable" (e.g. output a tag with inner content which is _not_ XML escaped).

There's no such thing in this case though, there's a single layer of tags with escaped content inside (the example document uses CDATA, but as others have noted automated generation is not a good use case for CDATA)

Agreed; just saying in general. Also, I would guess the reason they used ERB here is simply familiarity, not any type of reasoned decision.