Biblical tradition assigns authorship of the first five books of the Old Testament to Moses himself, and Elder McConkie, along with many Latter-day Saints both in and out of Church leadership, read the Pentateuch as a literal historical record. References to Aaron within that record have generally been read as a reliable, factual chronicle, which allows Mormon Doctrine to give us definitive details about Aaron’s life, his significance, and his relationship to the modern Church, notably the equivalence, in Elder McConkie’s words, to his functioning in the same way as the Presiding Bishop in The Church of Jesus Christ of Latter-day Saints. In that worldview, the ancient Church and the modern Church functioned along the same organizational template, and Aaron would have been right at home speaking at a modern General Conference about the temporal needs of ancient Israel.
However, as Wikipedia
notes, "Biblical scholars today agree almost unanimously that the Torah is the work of many authors over many centuries.” The probability that it is the kind of journalistic account that Elder McConkie assumed it to be is extremely remote. Indeed, most scholars argue that the the Old Testament patriarchs were mythological figures, and it is now conventional wisdom to say that Moses not only didn’t write the first five books of the Bible; he didn’t exist at all.
And if Moses didn’t exist, then, of course, neither did his older brother Aaron.
Latter-day Saints, on the other hand, cannot simply write off Moses and Aaron as fictional characters. The Doctrine and Covenants describes a visit by Moses to Joseph Smith at the dedication of the Kirtland Temple to give the modern Church "the keys of the gathering of Israel from the four parts of the earth, and the leading of the ten tribes from the land of the north.” ( D&C 110:11) To say that Moses was simply an inspiring legend, as much of the world now does, is to negate the Kirtland Temple dedication and strike at the heart of one of the crucial founding moments of The Church of Jesus Christ of Latter-day Saints.
So Latter-day Saint revelation makes it clear that Moses was a real person. And if Moses existed, Aaron existed, too.
That said, the fact that Latter-day Saints teach that Aaron was, indeed, an actual human being does not mean that one must also accept Elder McConkie’s rigidly literalistic view of the scriptural account of his life. There is a great deal of middle ground between Old Testament as fable and Old Testament as journalism. If stories of Moses and Aaron were kept alive through oral traditions for hundreds or even thousands of years before being written down, it is inevitable that those stories mixed the figurative with the literal in ways that were untroubling to a pre-modern audience. The value of what we know of Aaron is in the lessons drawn from the account of his life, not from the historical reliability of his curriculum vitae.
In fact, there are undoubtedly Latter-day Saints who do not believe in a historical Aaron, yet they accept the example passed down through the generations of the stories told about him. Such a position represents a minority view in the Church at large, and Elder McConkie would likely take a dim view of it. But as of 2020, belief in a completely literal Old Testament is not a necessary requirement for full participation in the Church. As such, members are free to have a wide latitude of opinion about Aaron, Moses, and the other people mentioned in the Old Testament.