^Actually, it's the lack of the comma that makes the joke funny. It shows how a writer who meant to say one thing ended up saying something totally different because they forgot the comma. If the hypothetical writer also chose not to capitalize the name of their uncle then it would look like they meant to write "jack off a horse" and that wouldn't be funny. The joke is the unintended mistake.
Oh and to the mods I think this still on the topic of doping (in writing not cycling but close enough) so it's on topic.