Sloppy writing again, but to give Puzo his due: Maybe Enzo had ust gotten off work, dressed formally for the visit, and, being a foreigner, didn't know that US hospitals had visiting hours.
Did Enzo tell Sonny that he stuck around? Or did Sonny hear it, perhaps from Detective Phillips, who was with Mac at the scene and was on Sonny's payroll? Or did Michael tell Tom after he woke up from having his jaw wired?