I'm not sure anyone is claiming Seinfeld invented the words but he presented them in newly humorous contexts that become widely used.
Shrinkage became a word used in a comical attempt to quash rumours of small penis size as well as to describe what happens when men swim in cold water. In the time when Seinfeld was popular then the word shrinkage used in a certain way would have been understood in these contexts and wouldn't have resulted in a HN-style overly literal interpretation and the question "what shrunk?". (speaking of overly-literal interpretations I feel the need to clarify that i'm referring to people who were vaguely up to date with popular culture).
Regifting isn't a literal description of giving a gift you received to another person. After Seinfeld's use it became a humorous euphemism for cheapness and tactlessness.
Shrinkage became a word used in a comical attempt to quash rumours of small penis size as well as to describe what happens when men swim in cold water. In the time when Seinfeld was popular then the word shrinkage used in a certain way would have been understood in these contexts and wouldn't have resulted in a HN-style overly literal interpretation and the question "what shrunk?". (speaking of overly-literal interpretations I feel the need to clarify that i'm referring to people who were vaguely up to date with popular culture).
Regifting isn't a literal description of giving a gift you received to another person. After Seinfeld's use it became a humorous euphemism for cheapness and tactlessness.