Thanks, Jeanie, that's a good question:

It doesn't. It just means that this geological formation is not evidence made by a biblical flood.

The problem with proving there wasn't a flood is that you cannot prove a negative. You can't prove that pink unicorns don't exist, for example, because you have not seen all of the universe. This is why science sticks with theory as an explanation of evidence, but held tentatively and discarded when invalidated.

Example:
(1) observation: no pink unicorns have been observed.
(2) theory: there are no pink unicorns.
(3) invalidation test: find a pink unicorn.

Note that theories are often stated as assumed truths in order to form invalidation tests, but this doesn't mean that they are true.

Getting back to the biblical flood and the Grand Canyon, we can show that the formations in the Grand Canyon are not consistent with flood flows, we can show that the formations in the Grand Canyon are consistent with long term normal stream erosion, however the biblical flood could still have occurred before the canyon was formed, before the ridge was uplifted, or before there was a river there at all.

Enjoy.