The value of a stratified deposit in this particular argument is that it was not written by anybody.
Almost everything Europe learned about Mexica ritual came through people with a stake in the answer. Andres de Tapia rode with Cortes. Bartolome de las Casas was arguing, against Juan Gines de Sepulveda, about whether the conquest was lawful at all, and accounts of sacrifice were the material of that argument on both sides. Bernardino de Sahagun assembled the Florentine Codex from Nahua informants, in Nahuatl, decades after the city fell, under the eye of a Church that had reasons to want the old religion to look a particular way. None of these are worthless. All of them need to be read as arguments.
An offering cache is not an argument. It contains what it contains, at a depth that can be dated, and the person who buried it was not addressing posterity. When the project finds a decapitated cranium in a sealed deposit under a known construction stage, that is a fact about Mexica practice with no European hand between the reader and the evidence.
The limit is equally worth stating. Deposits establish that a thing was done and roughly when. They do not establish how often, and they cannot on their own tell you what the participants understood themselves to be doing. For that the colonial texts remain necessary, which is why the honest position is neither to take them at their word nor to throw them away, but to say for each claim which kind of evidence is carrying it.