The answer has several aspects:

First: As al-Qādī explained, his words,

“Unless you become angels,”

expressed his assumption that angels were better than the two of them, just as he assumed that he was better than Adam—and he was mistaken. His words,

“Or become immortal,”

were based on his assumption that they would prefer immortality because it entails freedom from illnesses, sicknesses, pains, afflictions, and death. Such is the condition of anyone who lives eternally in Paradise. This was not a statement of superiority over the prophets. Do you not see that the maidens and youths created in Paradise live there eternally, yet are not superior to the prophets?

Second: An angel is superior in certain respects, and immortality was likewise more desirable to them, so they inclined towards it.

Third: Their condition at that time was an initial state, not their final state. For in their final state they have attained immortality, with no restriction inherent in it or accompanying it, and no cessation thereafter. Likewise, in their final state they reach a condition superior and more perfect than the angelic condition they initially sought. This is clear.

The fourth argument: Allah Most High says:

“Allah chooses messengers from among the angels and from among humankind.”

He mentioned the angels first, and one begins only with the best and noblest, followed by the next best and noblest, just as He did in His words:

“Those will be with the ones upon whom Allah has bestowed favour: the prophets, the utterly truthful, the martyrs, and the righteous.”

Thus, He began with the most perfect and the best.