"when you are walking your smaller toes do not press down as much as your big toe does.

to me this is more evidence that this particular cross section is not a carved fake."

This is an explanation that doesn't explain the observation. The fact that the big toe presses harder than the little toes would explain (if it were observed) a less pronounced feature under the little toes than under the big one. But that isn't what we observe. We observe the feature under the ridge BETWEEN the toes. Your explanation doesn't explain that. It also doesn't explain why the delineation of the feature doesn't seem to vary with the depth. The "substrate pressure line" seems absurd simply because it doesn't explain the observation.

Of course, I'm not a geologist. However considering that limestone is pretty porous stuff, the algal explanation seems a pretty good explanation. As one of the authors of the original paper is an actual PhD geologist, it seems likely he would be familiar with this kind of thing.