Textual: Mormon is taking a very literal expansion of the promise the Lord makes to Mosiah:
Mosiah 28:6-7
6 And king Mosiah went and inquired of the Lord if he should let his sons go up among the Lamanites to preach the word.
7 And the Lord said unto Mosiah: Let them go up, for many shall believe on their words, and they shall have eternal life; and I will deliver thy sons out of the hands of the Lamanites.
Since the Lord has promised to deliver the sons of Mosiah out of the hands of the Lamanites, and since Ammon was in a position to be killed twice, and was not, Mormon concludes that he could not be killed. It is important to remember that this is Mormon’s conclusion, not any statement made by Ammon, and probably nothing that would have occurred to Ammon.
There are many modern occasions where we believe that our missionaries have a similar promise of deliverance, and the world has sadly had other ideas. The promises of the Lord are for the Lord to uphold and understand. Ammon did not imprudently place himself in danger simply because he had a promise of deliverance. Neither should we, under a promise of deliverance, test the validity of such a promise by intentionally placing ourselves in harm’s way, believing in an Ammon-like invulnerability.