This question in itself has a lot of different things to address. The scriptures have obviously been corrupted as the Quran attests to
Do you covet [the hope, O believers], that they would believe for you while a party of them used to hear the words of Allah and then distort the Torah after they had understood it while they were knowing? Quran 2:75
So woe to those who write the "scripture" with their own hands, then say, "This is from Allah," in order to exchange it for a small price. Woe to them for what their hands have written and woe to them for what they earn. Quran 2:79
The Christians should judge each other according to their own books laws when having legal disputes and this has nothing to do with the Quran or rejecting Islam.
The Quran attests to the veracity of the original books that were given to the previous prophets and not the corrupted versions that exist today.
This person who wrote this obviously does not understand the Quran and probably has never read it.