Even if the US is right about their affiliation, Borzou Daragahi at the L.A. Times complicates the picture by describing the Quds Force as "an elite intelligence and paramilitary organization," and the Revolutionary Guard as "a parallel army" that "controls Iran's borders as well as its foreign policy toward neighboring countries such as Iraq and Afghanistan." [My italics]
The biggest concern on the US side, among other things, seems to be that Quds is helping to supply advanced armor-piercing IEDs to Shiite militias, so the best question is not who the five Iranians were, but what they were doing: Directly helping militias attack US forces, providing consular services, or gathering intelligence? And if the latter, the next question is: what sort?
[Jan. 31st edit: added "providing consular services"]