I agree with Cassius that it would not be fruitful to peruse who is lying - Apion or Josephus. There are may instances of Josephus were he is proud of his deceit, which is for him a virtue when used as in instrument in his success.
I would be avoid taking at face value any claims of Jewish ritual cannibalism or blood-drinking or inferring that the metzitzah b’peh is a vestige of this.
Even if we believe Josephus (who denies that Greeks were ever kept as prisoners in the temple and eaten), Josephus does admit to other instances of Jewish cannibalism, including eating children (although not ritualistically, but out of hunger) which horrified the Romans. For example:
Flavius Josephus, The Wars of the Jews, Book VI, Whiston chapter 3, Whiston section 4
"There was a certain woman that dwelt beyond Jordan, her name was Mary... ....snatching up her son, who was a child sucking at her breast, she said, "O thou miserable infant!... ...Come on; be thou my food." As soon as she had said this, she slew her son, and then roasted him, and eat the one half of him, and kept the other half by her concealed. Upon this the seditious came in presently, and smelling the horrid scent of this food, they threatened her that they would cut her throat immediately if she did not show them what food she had gotten ready. She replied that she had saved a very fine portion of it for them, and withal uncovered what was left of her son..."
The idea that Apion was Epicurean is very appealing, but it seems there is no direct evidence.