Aminadab does recognize what is going on. Note that he does not see the angels, he sees only the lighted faces and the attitude of conversing. Without a background in the religion of the Nephites, it would be unknown with whom they conversed, and this is precisely the reason the Lamanites ask the question. Aminadab’s background in Nephite religion taught him the basic symbols and religious language and ideas, so he is able to interpret that they were speaking with angels.