That is a definite possibility but we don't even know if there was a plot of any kind.
This was a campaign event and a bad shooting would have destroyed President Trumps chances of re election.
And President Trump is not currently the President so we have no idea what the rules of engagement were. Neither do we have any idea of any instructions that President Trump may have given to his security team about avoiding a bad shooting .
It would have been prudent but very bad optics to have the SS team drag Trump off the stage if they knew a threat was near by.
The prime witness in the media did state that the shooter was not visible from the over watch marksman's line of sight from their location.
Until more info is available, I'm going to ascribe this one to the Dual Golden BB - that there was some sort of near act of god crazy chain of events that allowed the shooter to get in place and shoot and even more incredible act of divine intervention that saved Trump from the bullet by the width of a soda strew. Time will tell.
The problem with that analysis is two key facts:
(1) Apparently there was at least one SS/police drone that would have seen the shooter. So much for line of sight issues.
(2) The drone intel of a guy on the roof with a gun is more than enough for SS to get President Trump ducking and then off the stage in less than a second.
Somebody in the communications chain wanted President Trump dead.