I'm not sure this affects the argument. Even if c were to change, creatures made of atoms would still have to be in conditions where their velocities relative t

I'm not sure this affects the argument. Even if c were to change, creatures made of atoms would still have to be in conditions where their velocities relative to the objects around them are low compared to c, otherwise they would disintegrate on contact. There are other assumptions about atoms that go into this of course, but the goal of the article is to explain why any creatures made of atoms would think of the speed of light as very fast compared to the interations they're accustomed to.