No one really knows one way or the other.
But ask yourself this: Why would any up and coming Navy officer or SEAL in the DC area attend this event in an unofficial capacity knowing the place is crawling with FEDS just itching to bust any one for looking cross-eyed at them.
Not to mention the facial recognition hardware. I was at a Tea Party rally on the Mall back in '09 and some agency had a sophisticated Hi-Lift type trailer that went up about 50 feet and slowly scanned the crowd in a 360. Every person who was there has a hi-def photo of their mug in a database somewhere.
Attendance at an event like this would be a sure career killer for any Gung-Ho serviceman.
Ergo, they almost certainly are FIBs.
You make a good point. Whether they are FBI, or some other flavor of civilian/military type, if they were not there for an official assignment, they would be in trouble.