This policy-rule-law is to deconflict any issues if there is another agency present, otherwise, the unidentifiable agents appear to be quasi-organized active shooters. Other law enforcement agencies, private security officers, or even law abiding conceal carry permit holders may take lethal action.
I've been involved is several incidents action plans and I've never seen such a cavalier attitude towards displaying proper uniforms and identification, or the casual description or blanket permission to engage in lethal force without very specific orders and chain-of-command-decision-making protocol. I've always seen a robust statement regarding when to employ less-than-lethal use of force.
These cowboys were itching for a conflict.
You bring up an important point-
apparently Mara Lago is HEAVILY guarded by on grounds security too- was the FBI hoping that the security would not know they were fbi because of lack of identifying markings and try to detain them, causing a possible incident for the use of deadly force?