I believe this to be the correct translation. So, why wouldn't G-d just strike them down with lightening bolts or something, if he can do anything? Why does he insist on having an army to defend the mountain so no one can see him?
The issue derived from the verse
No hand shall touch it, for he shall be stoned or cast down; whether man or beast, he shall not live. When the ram's horn sounds a long, drawn out blast, they may ascend the mountain.
Seems to be not one of defense, but of legal culpability as the two methods of death refer to 2 technical types of death penalty. As Rashi indicates,
From here [it is derived] that those liable to death by stoning are [first] cast down from the stoning place, which was as high as two heights [of a man]. — [from Sanh. 45a] cast down-Heb. יִיָרֶה, shall be
The Chizkuni brings out a different reason for specifying these 2 types of death penalty:
“you must not touch him;” (the offender in order to pull him back) rather he shall be killed by stoning or by shooting arrows at him, but he must not be touched by human hand in order to be brought before a court of law to be dealt with. The Torah adds therefore that his death should occur without humans touching him. This prohibition applies equally to the Israelites’ beasts. If you were to ask why ascending the mountain is a worse offense than killing such a person by stoning him while ascending? The reason is that if the offender were to be killed through human action, the party touching that body, dead or alive, would also be considered as having personally touched that Mountain, whose holiness is such that it must not be touched by human bodies.
Other commentators (such as the Daat Zekeinim and the Rashbam) agree -- this is about the technical form of the death penalty imposed by the court, not a defense mechanism.