First: Why do you think needing the tidal calender?
However: My idea is that the tide table applies for those who want to walk from the main land to Le Mont-Saint-Michel THROUGH the bay, so over land and not over the new walkway which also is in use by the free ‘Passeur” shuttle bus. The walkway will only be flooded several hours a year marked with red on the tide table and the island will obviously not accessible.
In case you want to walk through the bay you have to know about the tidle movement as opcoming water can take you by surprise and for that reason doing this without a guide is very dangerous. I think the two hours is necessary to arrive before the water comes up as parts of the bay can already start to flood in an early stage, but have to say that is my idea and not 100% sure about it. But know that even if land is flooded a few centimeter it remains dangerous as deep pits are not visible anymore, you will risk braking a leg if you step into one or you can get stuck in the mud with even the risk of drowning. Some pits are covered with vegetation, so not visible and dangerous anyway, so again always use a guide in this case.
You don’t have to think about this using the walkway like the majority of the visitors do.