There’s still a difference among the surfaces, but what the writer was getting at was that when, for example, comparing grass and clay, the differences aren’t as extreme as they once were. I’m not sure what changes exactly have been made, but everything has been brought to a middle ground that favors the skill set described in the article. I guess maybe you keep the grass a bit longer and that slows up the court. With clay, it does appear as if surfaces just aren’t as “cakey” as they once were. So that speeds it up a bit — again, closer to a middle ground. Hard courts have always been generically described as “hard” so difficult to say what changes over the years have affected speed there.