What you're seeing isn't simply mutation. The evolution of those mutant strains requires that they survive and reproduce. If weaker bacteria are able to eat the nutrients on the plate, that leaves nothing for the mutating strands that will survive. The team called it "a powerful, unvarnished visualization of bacterial movement, death and survival; evolution at work, visible to the naked eye."
What's phenomenal outside the research is that they've created the conditions to present their experiment in a way that's accessible to someone who didn't, say, graduate from Harvard. They dyed the agar jelly black to make the bacteria visible and heated the lid of the petri dish to keep it dry and free of condensation, among other moves with an eye on the cinematic effect of the experiment. Those efforts make it easy to see evolution at work, even for those who might think science is something you "buy into."