The text plays on the re-framing of the concept of hair loss by substituting the negative connotation of 'losing hair' with the positive spin of 'gaining more scalp'. It creates humor by juxtaposing the typical concern about hair loss with an absurdly optimistic perspective, but it does not exploit different meanings of a word or similar-sounding words.
You ' re not losing more hair , you ' re gaining more scalp .