You don't need to pull someone over and do something which is illegal for a civilian. If you just modify a car to look like a police car and driver around with it, then that is likely enough. Similar, even if you only walk around in a police uniform with a fake badge and don't do anything more, it will likely end up badly.
Context matter, which is why intent and result are part of the equation. If people are being fooled into thinking someone is a police, and someone had the intent to fool others (maybe with the intention to create false association), then that is likely fully enough to get a person charged with impersonating a police officer.
That's the equivalent of stolen valor, which is a separate issue of fabricating past accomplishments. The commentator did not claim they were a current CIA officer.
Looks like the worst they've done is made some screenshots https://twitter.com/ALT_uscis/status/833455581734326272 which don't look obviously wrong.
This is not the same as pulling over someone and pretending you're a cop. Context matters.