I don't know where the water/tides were back then, nor where this site is precisely in relation, but I would guess that a simple mudslide from the soft, nearby cliffs could have covered/preserved them.
SO, what must have happened at that place to preserve these footprints for literally thousands of years? No weathering, no erosion, no waves, no wind, no rain, no entropy at all?..................
About 40 billion people have walked the Earth. Imagine what a tiny fraction 87 must be of all the footprints ever left by anyone.
I don't know where the water/tides were back then, nor where this site is precisely in relation, but I would guess that a simple mudslide from the soft, nearby cliffs could have covered/preserved them.
Serendipity.
But your premise is incorrect with a lot of them from the photos. A fair number are clear, while most were clearly affected by erosion or whatnot.