No, it isn’t. Which means that defining an early Christian apostasy as the loss of priesthood authority doesn’t tell us anything, even in a Mormon framework, about the apostasy as a historical event, or why it occurred, or what it consisted of. If we define the apostasy as the withdrawal of divine approval and loss of authorized inspired leadership, then there is no way to identify the apostasy with any set of historical events.
This is partly due to the nature of history. For a Mormon understanding of priesthood, loss of authority implies either 1) a gap in the succession of authorized ordinations, for which there is no documentary evidence; or 2) the withdrawal of divine approval from the duly ordained, which is a spiritual event entirely outside the purview of historical inquiry. That we call John rather than Mani a true prophet is not a matter of weighing historical evidence, but a matter of our present belief.
But the ahistoricity of authority is also partly due to our understanding of priesthood. One careless way to define apostasy is as change in belief, in practice, or in scriptural texts from an earlier state. But the Mormon concept of priesthood authority undermines these arguments: belief, practice, and scripture are all liable to authorized change through the priesthood itself, via the process of continuing revelation. Observing changes in scriptural texts over time therefore tells us nothing about the presence or absence of priesthood authority behind them; all we can do is state whether or not we believe such changes to have been authorized or not. (There is also the even sloppier argument from difference, which dispenses entirely with the chore of determining what early Christianity was actually like by assuming that it was all but identical to current Mormon practices and beliefs, so that anything differing from our present condition is labeled as apostate.) A final variant is the argument from revulsion: namely, that people calling themselves church leaders believed or said things that no truly inspired leader could ever do or say. Unfortunately, canonical scripture raises the bar rather high in terms of what prophets acting under divine inspiration might do at any given moment: attempt to sacrifice their child, lie, decapitate a helpless enemy, wed a shameful woman, advocate genocide. Even this variant, the “Donâ€™t you know what Tertullian did?” argument, reduces to the simple assertion that Tertullian did not have priesthood authority, rather than serving as historical evidence that priesthood authority had been lost.
Note that this is only a problem if you want to find historical evidence for an early Christian apostasy. You can believe that such an event took place (I do) and that a loss of priesthood authority was involved, and even see the consequences of the apostasy all across history, and all that is lost is the possibility of mounting historical arguments to support your view. As an alternative, you can define the apostasy in other terms to preserve its historicity, or attempt a slightly different approach altogether. But looking for historical evidence of a loss of priesthood authority will probably not get you an inch beyond the claim that our church is true, and others are not.