The resurrection myths do not disprove Christ’s resurrection, rather they point to it. The ancient myths are merely fractures images of the promise God made to Adam and Eve after they fell and were cast out of the Garden:
Gn. 3[15] I will put enmity between you and the woman,
and between your seed and her seed;
he shall bruise your head,
and you shall bruise his heel."
Thus, all the descendants of Adam and Eve had this primordial promise of the coming Messiah. The great pagan poets and philosophers had enough inspiration from the Holy Spirit to write the truth but in mythological form. Mythology is not the same thing as fairy tales (and even fairy tales can relate truth). Rather, mythology is merely a literary form, like poetry or prose, that tells a story, true or imaginary, that conveys truths, cultural, religious, whatever. Mankind had the promise given to them which they passed on through stories and mythologies. It came physically true in the person of Jesus Christ. That is what Justin Martyr was referring to.