The truths contained in religion (like the truths in wisdom or ethics) seem to be transcendent. These truths span cultures and times and ethnicities and places. They are not localized and particular but rather universal and transcendent.
The statement, “The result would just be we have no reason to consider Christian revelation to be true,” would not seem to follow, even if aliens were the source. There might be particular aspects of the Christian religion that would be false. But the transcendental truths stand on their own—humans have intrinsic dignity, there is something amiss about this present world that needs correction, the desire of the Creator is to draw all men to himself in an afterlife…these and many others besides still obtain, no matter the source of the religion.
Also, there is something bizarrely akin to an “argument from silence” about your suggestion here. There aren’t testimonies of religions that claim aliens as their source. You’re arguing from a logical possibility, with no testimonial evidence, which is an argument from what isn’t there (from silence), right?
Yes? No? Maybe?