Characters
Hi, and welcome to the future. San Dimas, California, 2688. And I’m telling you, everything is great. The air is clean. The water is clean. Even the dirt is clean.
— Rufus

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
This is a cheerful sales pitch for a spotless future, with everything described as great. The joke builds through the list of clean things and lands on 'even the dirt is clean', which is so oddly overdone that it sounds comically unbelievable.
Context reviewed