The joke is that people wanting to write Python like Java - with an implicit reference to an object's instance - would attempt to import Java's `this` keyword.
Instead of getting Java-like behavior, they're presented with the Zen of Python. You wouldn't be wrong to think that it was as much a commentary on other popular languages of the day as it was a guiding ethos of Python itself.
"Beautiful is better than ugly. Explicit is better than implicit."