3 witnesses of Christ here.
And it came to pass that Ammon went and inquired of the Lord, and the Lord said unto him: Alma 27:11–12
The Lord answers Ammon's prayer and commands him to lead the people out, promising 'I will preserve them.' Like Jehovah leading Israel out of danger, the premortal Christ guides His covenant people to safety.
How we know: Jehovah is the premortal Christ (Abraham 1:16; D&C 110:3–4)
And they were among the people of Nephi, and also numbered among the people who were of the church of God. And they were also distinguished for their zeal towards God, and also towards men; for they were perfectly honest and upright in all things; and they were firm in the faith of Christ, even unto the end. Alma 27:27
The people of Ammon are remembered for being firm in the faith of Christ all the way to the end. Enduring to the end in faith on Christ is part of the doctrine of Christ.
How we know: the verse itself names Him
And they did look upon shedding the blood of their brethren with the greatest abhorrence; and they never could be prevailed upon to take up arms against their brethren; and they never did look upon death with any degree of terror, for their hope and views of Christ and the resurrection; therefore, death was swallowed up to them by the victory of Christ over it. Alma 27:28
These converts had no fear of death because they knew Christ would conquer it and raise them up. For them, death was 'swallowed up' by Christ's victory, which is why they could lay down their lives rather than kill.
How we know: the verse itself names Him