Enough seriousness, you could use some of these:
Origin of the term "Doctor"
Anyone who says that an MD is the only degree which should be referred to as "Doctor" should understand that using this title to describe someone with a Doctor of Philosophy degree was a practice in place centuries before the term began to be used for the Doctor of Medicine degree.
From Britannica:
The term doctor, from the Latin verb docere, meaning “to teach,” emerged in the Middle Ages, when it was used to describe theologians who were qualified to teach religious doctrine. By the 14th century the title had been expanded to refer to all those who received a Doctor of Philosophy (Ph.D.) degree. In the 17th century, with the growth of respect for medical training, medical schools, primarily in Scotland, began to address physicians as doctors. Previously, physicians had been excluded from this title because their training was considered to be professional (preparing students for careers) rather than the kind of advanced discipline-specific learning offered via a graduate program.