How do atheists and evolutionists explain the flaw presented in this statement?

There is more than radioactive dating available and the best way to measure a rocks age is to use a few techniques, that bit is a fact. Although I am not 100% on this I think the answer you need is that when a rock is formed you would have brand new carbon atoms, carbon then breaks down over the years at a very steady rate, as the carbon becomes more and more 'unstable' then you measure this by radio carbon dating. Does that make sense to you?