Teddy Kennedy also had a February birthday—so there would have been 5 Presidents born in February if he had been elected. It may be that alcohol prevented that, if he was drunk when he drove off the bridge at Chappaquiddick (he claimed that he had had only 3 rum & Cokes before driving).
You might remember that when Teddy Kennedy ran against Carter, Carter kept saying (almost daily) that he didn’t think Chappaquiddick should be an issue — thus making it one, while looking like he wasn’t.