Potential form with を


My question is pretty straightforward (I think): I was reading a story from an
app for learning Japanese and came across this phrase:

> そしてその友達の輪を、どんどん世界中に広げられるといいですね

So far, I have only studied basic grammar, and my primary learning source has
been Tae Kim's guide. There he states that I shall not use を with the
potential form, and thus follows with examples where を is "substituted" with が
or は. So, is my quotation an exception? An error? A colloquialism?

