That assertion is untrue: the Canadian Shield is just one example of a large (half of Canada!) region that's pretty much devoid of any sediment at all. The rocks in the whole region are metamorphic or igneous, and I defy any creationist to get Flood sediments metamorphosed and cooled again in 4500 years!
And in places like here in West Texas, where the geologic column is mostly sediment, it can hardly be construed as flood sediment. 6500 feet beneath Snyder there is the Canyon Reef, a coral reef about 300 to 600 feet thick. Reefs don't grow that thick in a year or two. The 6500 feet between the Reef and me is made up of multiple layers of limestones, dolomites, and shales, with a little sandstone up near the top. Limestones like these form by slow "rain" of shells of marine plankton from non-muddy, calm seas; the dolomites form when rainwater (falling on other than a sea, obviously) carries dissolved magnesium down to percolate through the previously laid limestone. Not a quick process, either, and there are limestones above dolomites - it couldn't have all happened "post-Flood." This whole area was alternately a shallow sea and dry land, several times as each, of very long periods of time.