The question shouldn't be "Why God exists in all people mind?" It should be "Why does God STAY in unbelievers' minds."
How can God remain in the minds of unbelievers if it is not conscious? You wishful thinking. In my mind there is no God and him there is no place. There are a lot of real things for the mind to be distracted by stories.The guy is delusional. Ignore him. He knows what God is thinking, never mind atheists. He thinks he got the "answer" everybody is missing. His delusion runs deep, he creates this God fantasy to best suit his warped world view.
His God concept is very flexible, he adjusts it as required. If tomorrow scientists discover ways to travel to other galaxies at warp speeds, he will claim that God decided to give us this information so that humanity can live to see another day. He will claim that it is thanks to God (not the scientists) that we can colonize another planet (that God prepared for us), etc. He will find some verse in the Bible (or write a new version of the Bible) and say that this "so-and-so" verse means that God prepared other planets for people to live on....
There is no limit to his delusion (God). Sky is the limit for this guy's God delusion.