Haha it's ironic that they're using bad grammer. The "it" refers to the indefinite pronoun, so the sentence is just saying that when you have an "it", it can be unclear what that "it" is a substitute for. When in doubt, I try to replace the "it" with what I think it is pointing to see if the sentence still makes sense.
An Indefinite Pronoun is not specific about the thing to which *the indefinite pronoun* refers to.
This makes more sense than:
An Indefinite Pronoun is not specific about the thing to which *the thing* refers to.
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