Apparently they did know he wwasnt scheduled to be there...
“Should FPOTUS [Trump] arrive at MAL [Mar-a-Lago], FBI MM EM and OSCs will be prepared to engage with FPOTUS and USSS Security Team.”
I think people are taking the work “engage” out of context and blowing the lethal force warning out of proportion. This looks to be a fill in the blanks planning document. This is probably what they are supposed to do every time there is a high risk search warrant issued using federal agents.