Speaking as someone who's split ~half my time between Leipzig and Berlin for the last 5 years, this is not true.
Leipzig's club scene is an extension of its university population. It's younger, straighter, whiter, and about two orders of magnitude smaller. People visit the clubs while they're going to school there, then they graduate and move elsewhere. Often to Berlin.
Why does this illusion exist? Because Leipzig is about an hour away from Berlin by train. Berliners visit for a weekend and think "wow, it's like Berlin in the 90s! Still cheap! And look at all these cool young kids at these scrappy clubs -- so that's where the underground has gone!". Then they go back to Berlin and spread the word to credulous out-of-towners, who go on to repeat this truism to people who have never visited either city.
In reality, Berlin's club scene -- both "mainstream" and "underground" -- dwarfs that of any other city. Nothing short of an asteroid hit is likely to change that.