How many documented cases are there of narrowboats in tidal or wide-ranging berths? Given that the "normal" of a non-tidal river is normally the lowest it would be, it would be very unlikely you'd want to "go lower" than what you cruised to, moored up at, then would rise again then depart. For example, think of a flood-safe river mooring. You may well want/need to moor to it in flood, but you'll also want to be floating and depart from it later. Hence why they are purpose-designed to go up and down and allow mooring/unmooring in all conditions.
And.....a narrowboat simply isn't designed for waters in the same way as a sea-going boat (some of which ARE designed to work well in tidal moorings). Its inlets/outlets are the lowest of any boat, I'd not want to be taking chances. Having said all that, the mud will definitely loosen up once the dried-out mooring is flooded again, so its unlikely to hold with much force over the boat's buoyancy, so I'll put it down as "myth" too!