“Probably was preserved by anoxic mud and water so polluted that almost all decay organisms cannot live.”
Anoxic conditions are the key, as wood has been recovered from clear trout and salmon waters where pollution is not a significant issue.
The decay organism I mostly had in mind with my comment about pollution were ship worms, which live in brackish and salt water. I remember reading that improving water conditions caused century old piers to suddenly collapse in NY in the 80’s, because the improving water conditions allowed them to thrive.