Why are Jim and Della called magi?

Mr. and Mrs. James Dillingham Young, or Jim and Della, are called "the magi" in the final sentence of O. Henry's short story. It is explained at the conclusion of the story that the magi were the "wonderfully wise men" who gave the very first Christmas presents when they bore gifts to Christ in the manger. Since they were wise men, their gifts were undoubtably wise, too, and it is suggested that the spirit of their love and generosity are something to strive for in the gift-giving season.
In the story, Jim and Della are a young married couple who, despite pinching pennies from their meager salaries, find they don't have the money saved to buy the gift they each think the other deserves. Della finds she has "only $1.87 to buy a gift for Jim. Her Jim. She had had many happy hours planning something nice for him. Something nearly good enough. Something almost worth the honor of belonging to Jim." We learn that despite their humble living circumstances, each character has something that is precious and of much value to them. Jim owns a watch that has been passed down to him by his father, who received it from his father. Della has beautiful brown hair that reaches below her knee. O. Henry compares these items to the riches and jewels of kings and queens, and the items seem priceless due to their rarity and sentimental value. The comparison of them to jewels, juxtaposed against the repetitive use of grays and browns in the description of their clothing and home, suggest that these items bring them much joy despite their challenging and modest circumstances.
However, a major theme of the story is love, and despite their love for these items, their devotion to the happiness of each other is stronger. Della sells her hair to buy the perfect chain for Jim’s watch, and Della soon learns that Jim has sold his watch to buy hair combs that Della has long admired. These seemingly priceless items actually pale in value when compared to their love for one another. "And here I have told you the story of two children who were not wise," the narrator tells us, suggesting that many readers might view Della and Jim as foolish, with both their sacrifices and new gifts rendered useless. However, the narrator quickly adds, "Let me speak a last word to the wise of these days: Of all who give gifts, these two were the most wise. Of all who give and receive gifts, such as they are the most wise." The moral of the story is that the true gift that both Della and Jim give each other is the sacrifice of something personally valuable in order to bring happiness to the other. Furthermore, Della optimistically tells Jim, "My hair grows so fast," and Jim "smiled" at seeing her gift. Rather than criticizing each other or mourning the loss of their treasures, both characters recognize the value of each other's sacrifice and are content. This knowledge and understanding makes them wisest of gift-givers, and, therefore, they are considered to be like the magi.
https://americanenglish.state.gov/files/ae/resource_files/1-the_gift_of_the_magi_0.pdf