This whole thing is intended as a joke, but if anyone does show up, no lethal force is going to be used. Lots of ways to control crowds that don’t involve gunning people down, especially with advanced notice.
In addition to being a general deterrent to keep fools away, the “deadly force” part of the signage is really for those situations where some nut is going in at top speed in his car or something similar where it might be an attempted terrorist attack and a quick decision has to be made. Groom Lake has a long history of people walking a few too feet too close and people just get shooed away or at worst arrested by the local sheriff.
They will be warned, escalating to being asked to voluntarily subdue themselves, escalating to being physically halted and restrained/incapacitated, escalating to death. They are not going to get anywhere near anything sensitive, or expose the extent of the security arrangements of anything sensitive.
This is not a couple of nutcase trying to wander onto a base and post a protest sign on the PX.
It also has a history of one idiot who persisted and was returned in a body bag.
You are extrapolating from a civilian perspective - none will set foot on the base, alive.
“Deadly force is authorized” are not merely words to scare idiots away.