No, they allegedly told the staffer that they didn't have their telephone repairman ID's. You're making an assumption that they didn't have any ID on them at all. The US Marshall will frequently effect arrest on behalf of other federal agencies. I wouldn't read anything into it, especially considering that the alleged crime occurred in the federal courthouse - where the US Marshall's have purview.
GSA also has “guards”. THey got past the guards. No notice in any of this who is guarding this particular building.