"But Moses kept his gaze fixed upon the cloudy pillar, and encouraged the people with the thought that the token of God's presence was still with them. At the same time he directed them to do all that human power could do in preparing for war. Their enemies were eager for battle, and confident that they would blot out the unprepared Israelites from the land. But from the Possessor of all lands the mandate had gone forth to the leader of Israel: “Rise ye up, take your journey, and pass over the river Arnon: behold, I have given into thine hand Sihon the Amorite, king of Heshbon, and his land: begin to possess it, and contend with him in battle. This day will I begin to put the dread of thee and the fear of thee upon the nations that are under the whole heaven, who shall hear report of thee, and shall tremble, and be in anguish because of thee.” PP 434.1"
"And the Lord said unto Moses, Fear him not: for I have delivered him into thy hand, and all his people, and his land; and thou shalt do to him as thou didst unto Sihon king of the Amorites, which dwelt at Heshbon."
The EGW passage directly quotes God's command to Moses regarding Sihon, mirroring the sentiment and context of Numbers 21:34, although it doesn't directly quote that verse. It's an allusion to the broader narrative.
These nations on the borders of Canaan would have been spared, had they not stood, in defiance of God's word, to oppose the progress of Israel. The Lord had shown Himself to be long-suffering, of great kindness and tender pity, even to these heathen peoples. When Abraham was shown in vision that his seed, the children of Israel, should be strangers in a strange land four hundred years, the Lord gave him the promise, “In the fourth generation they shall come hither again: for the iniquity of the Amorites is not yet full.” Genesis 15:16 . Although the Amorites were idolaters, whose life was justly forfeited by their great wickedness, God spared them four hundred years to give them unmistakable evidence that He was the only true God, the Maker of heaven and earth. All His wonders in bringing Israel from Egypt were known to them. Sufficient evidence was given; they might have known the truth, had they been willing to turn from their idolatry and licentiousness. But they rejected the light …
"But in the fourth generation they shall come hither again: for the iniquity of the Amorites is not yet full."
Ellen White directly references Genesis 15:16 in this passage.
When the Lord brought His people a second time to the borders of Canaan, additional evidence of His power was granted to those heathen nations. They saw that God was with Israel in the victory gained over King Arad and the Canaanites, and in the miracle wrought to save those who were perishing from the sting of the serpents. Although the Israelites had been refused a passage through the land of Edom, thus being compelled to take the long and difficult route by the Red Sea, yet in all their journeyings and encampments, past the land of Edom, of Moab and Ammon, they had shown no hostility, and had done no injury to the people or their possessions. On reaching the border of the Amorites, Israel had asked permission only to travel directly through the country, promising to observe the same rules that had governed their intercourse with other nations. When the Amorite king refused this courteous solicitation, and defiantly gathered his hosts for battle, their cup of iniquity was full, and G…
"The people shall hear, and be afraid: sorrow shall take hold on the inhabitants of Palestina."
Both passages describe the fear and dread that the surrounding nations felt towards Israel due to God's power. While Exodus 15:14 is a general statement, the EGW passage provides specific examples of events that caused this fear, making it a thematic parallel.