Blatant lie.
I'm sure God told Adam how he was created, which makes YEC as old as the first human. Certainly Noah's family believed in the flood, and they lived over 4,500 years ago.
The accusation of YEC being a recent concept is a lie, and those who state such are liars.
When the King James Bible was completed in 1611 it was presumed to be true as well.
False theories of millions of years of evolution is the recent theory, not YEC.
The only extreme view of Genesis is to call it mythology and lies. Obviously the early Christians believed it was true, since the first three chapters alone are referenced over 200 times in the New testament. Jesus taught that the Scriptures were accurate and suitable for instruction; that no jot or tittle would pass away until He returned. Since He spoke of Noah, Caine and Jonah by name, obviously He considered them real people.