I agree that's a chip shot with a service rifle or a tapco system but do the agents carry rifles or is it a pdw or subjunctive undercarriage jacket? I got passed by Laura Bush's motorcade once and they had a trunk monkey in the back of the Tahoe so I would assume something belt fed like a SAW but you don't often see the immediate detail with long guns. I am sure the are close.
I am not an expert in USSS tactics. For that matter, I'm not sure the USSS is an expert in USSS tactics, but If I have done VIP protection (in the Army, in a war zone, so a bit different, but still). If I were building an executive protection detail, the close in guys would wear the suits and pistols you so often see, and the intermediate guys would be in civies with PDW's concealed so they weren't 100% obvious, but the farther out guys? The folks 1-2 holes ahead of the VIP doing route clearance? If those guys aren't in plate carriers with duty rifles, you done *expletive deleted*ed up IMO. Not just for the response to a threat (like this one), but you are going to want a show of overwhelming force for anyone doing recon for a future attack.
JMO, USSS might have a different one.