I’m not sure, though, that this would make Christianity more testable, as the quote in the OP suggests. The point about miracles, surely, is that they are not testable? You can’t repeat the experiment. Or, if you do, you can’t draw any reliable conclusions from what you observe.
If we compare Islam with Christianity, in both cases the founding figure are set to have insights or revelations. We can’t really test whether they did or not. Or, if we accept that they did, we can’t test the validity, authority, etc, of the insights or revelations.
In both cases it’s also claimed that the founding figures then preached, gathered followers, established some kind of community, exercised a leadership role. Those claims can be tested, at least in principle, in the same way that we can test other claims about historical events - we can critically assess the sources, etc, and form a view as to how likely it is that the claimed events in fact occurred.
But, to the extent that Christianity also rests on claims about the Resurrection, that’s an additional element that, being miraculous, can’t be tested. We could use historical techniques to form a view as to how old belief in the Resurrection is, how widespread the belief was, and how early. But that doesn’t tell us whether the belief is actually correct or not.
Thus it seems to me that, all in all, Christianity is less testable than religions which don’t base their validity on publicly-attested miracles. In both cases we have (untestable) teachings which are accepted by the founding generation of the faith. In the case of Islam, the initial adherents are taken to have accepted the teachings because they found the teachings convincing or appealing, and because they themselves (a Muslim would say) were wise, humble and well-disposed - no supernatural claims there. In the case of Christianity, we have teachings being accepted by a first generation at least in part because they are convinced of the reality of the Resurrection, which is clearly an additional supernatural (and untestable) claim.