Just because someone believes MANKIND is only ~6k years old doesn't mean that he believes the EARTH is that young. The Bible gives the account of the creation of the "Heavens and the Earth" BefORE it begins talking about the creative days. The Earth's creation could have been BILLIONS of years before God turned his attention to the Earth.
So, according to EVOLUTIONISTS, the dating techniques are perfectly good to prove EVOLUTION right.
"Beware the sound of one hand clapping" -Stephen Meyer
And furthermore, wherever did you get the idea that the FLOOD KILLED ALL VEGETATION? What do you think Noah saw when coming out of the ark, a MOONSCAPE?