The rules are a bit fuzzy, and the recent ruling that "meeple" is invariable does not help either.
The Road Sweeper gift card allows you to score any incomplete road of your choosing, empty or occupied. As you mention, the active player seem to have to remove one of their meeples, if any is present on the chosen road. But the rules address the simplest case only even in the example, so there are many moving parts here, I reckon.
I sent a couple of questions to HiG about this. I hope they reply soon.
EDIT: This interpretation of the rules was wrong. The post below clarifies their real intent.