An unconformity is not "proof" of a long time period, but instead of an erosional event. Sediments laid down by water, uplifted, and later eroded and overlain by other sediments laid down by are perfectly consistent with the Biblical flood.
That is entirely impossible in the time frame of a biblical flood. It requires millions of years not thousands. The Old Red Sandstone was laid down 350 million years ago.
No prior assumptions are required to conclude that an unconformity is evidence of deep time. The physical evidence speaks for itself.