Keep in mind this whole assassination attempt scenario and its confusing responses fueled by the democratically controlled machine has shifted into the theater of the absurd and your question, because I wondered the same thing and after seeing the degree of 'flinching' taking place on the roof during the time the shots are heard, I'm going to offer this explanation.
To be perfectly clear, this opinion is mine and based on my life experiences that were shaped by:
1)A general distrust of most politicians and a dislike of democrats and
2)Learning rifle basics on Parris Island more than six decades ago; I was never a sniper.
Based on the aforementioned distrust and dislike of politicians, Democrats in particular and the unlikelihood of the degree of 'flinching', as seen in the video, happening when the shooter is designated a 'sniper' my conclusion is that the acknowledged lack of high level security onsite does not preclude the use of an actor or actors to provide the necessary sense of security for the occasion.
Think of all the calls for 'crisis' actors over the years, why not backfill necessary but unavailable personnel with actors with blanks in the rifle? Experience not necessary as proven with the 'flinch' caused by a finger on the trigger before he was ready to shoot?
Just another scenario for the theater of the politically absurd in the Land of the Free.
Not beyond the realm of possibility.