The miracle of splitting moon was witnessed by all the enemies of the Prophet Muhammad (SAWA) and was mentioned clearly in Quran in front of all the enemies including the idol worship pets of Quraish, Jewish and Christians people, and none of them challenged it. This itself proves that it was an undeniable fact.
An ancient India document quoted by Muhammad Hameedullah in his book ( Muhammad, The Messenger of Allah) mentioned that a king in Malabar southern India by the name of Chakarwati Farmas has witnessed the splitting of the moon and informed others and traveled to Makkah to meet the Prophet. He accepted Islam and returned back india but died in the way back and buried in a place called Dhafaar which is now in Sultane of Oman. This document is still existing in the British Museum according to this book under number 2872/152-173.
In short, if this miracle mentioned in Quran did not happen, you would have seen many enemies of Quran denying it, but no one at all denied it.