He was a firm believer in bleeding, and never let the constant failure of this treatment and resulting death of his patients keep him from tapping into the veins of his next victim. Also, he was an authoritarian, misogynist (even by the standards of his time), racist (more in keeping with those standards) and a raging ass who tried to drive other practitioners from the field. The quote you cite was probably opportunistic ass-covering. You should read up on who you cite.
Here's the first piece I found, it jibes with what I remember from other readings about him:
Benjamin Rush was a brilliant man (mentioned already in our previous essay). It’s quite funny how lightly some historians treat him, considering the probability that his medicine killed more people than it ever saved. Rush graduated from the college we now know as Princeton at the youthful age of 15. He then studied medicine as an apprentice under a Dr Redman in Philadelphia and attended the first course in anatomy to be taught in this country. In 1766 he went to Edinburgh, Scotland and received his medical degree in 1768. He took Cullen’s theories back to the colonies where he, according to Mary Gillet, "eventually modified Cullen's doctrines, which he had originally so much admired, and discouraged the study of separate disease entities by blaming all disease on excessive tension which caused disturbance in the blood vessels. By 1793, he was openly contending that there was but one single disease in existence." She goes on to say:
The method of treatment upon which Rush insisted with increasing inflexibility called for a low diet, vigorous purges with calomel and jalap, and bleeding until the patient fainted. Rush apparently did not hesitate to remove a quart of blood at a time, or, should unfavorable symptoms continue, to repeat such a bleeding two or three times within a two- to three-day period, it being permissible in his opinion to drain as much as four-fifths of the body's total blood supply. In time, Rush's system and treatment became, in the words of a noted medical historian and physician, "the most popular and also the most dangerous 'system' in America.
Rush later became the middle army’s first Surgeon General. However, the story of the Director General and Chief Physician of the Hospital of the Army, from July of 1775 through Oct of 1775, is much more interesting.