As Earth's only native guardian at present, the Sorcerer Supreme, The Ancient One, had long been classified as S.H.I.E.L.D.'s highest-level secret.
More precisely, when she first appeared and took away the little girl who had awakened elemental magic, footage of her was captured by satellites and nearby undamaged cameras.
Later, after confirmation from Balthazar, the Monk with Quasi-Divine Power, and Raymond himself, Nick Fury classified her as a final trump card capable of protecting Earth at critical moments, just like Captain Marvel. He had even considered recruiting her into the Avengers.
Unfortunately, the Sorcerer Supreme had no interest whatsoever in S.H.I.E.L.D.'s olive branch and could not even be bothered to respond. In the end, the matter was simply dropped.
Of course, Director Fury had not realized just how vast the gap between Captain Marvel and The Ancient One truly was.
In his eyes, both of them possessed unimaginable power.
It was like two people both scoring one hundred points: one had given it everything they had, while the other had only scored that high because one hundred was the maximum score.
If the two fought, Carol Danvers would probably die at the Sorcerer Supreme's hands before she even understood what had happened.
Even without using the Time Stone within the Eye of Agamotto, the latter had countless means of playing the former in the palm of her hand.
As one of the informed parties, Coulson did not know much about The Ancient One. He only knew that she led Earth's largest organization of magic users and ensured human society remained safe from the invasions of malicious Dimensional Demon Gods and all manner of Monsters. To a large extent, she was considered an ally.
It was simply that she had existed longer, possessed a more ancient history, and had greater seniority.