The process for finding a specific random point in a maze usually involves 75%+traversal. It doesn't matter if you're a single entity or not, as the slime mold is just effective running it all in parallel.

Even given the location of an exit, most solvers still end up searching most of the map before finding it.

Hey, uh... What do you think maze solving algorithms do?

You actually missed the real criticism, in that it's more or less a optimized maze search shortest path algorithm, which can be selected for chemically.

But uh, that's all biology is, including neurons. So the line still exists. The question isn't "Is finding the maze high intelligence?" It's "Where do you put the arbitrary line?".