The Zapruda film disputes that. You can see Kennedy grab his throat a few seconds before his head explodes from another bullet. Did the one bullet wait and then travel to his head from his throat? Whether you think that the shots were fired from the book repositary or the grassy knoll, it wasn’t one bullet.
The “single bullet theory” doesn’t say that only one bullet hit JFK. It says that the first bullet hitting him is also the one that hit John Connally.