[Source] She [Dr Jenny Pynt] pointed out a few other categories that often spell trouble, recommending against stools and other seats without backrests, at least as full-time accommodations, “because no matter how virtuous you are, you will slump.”
Virtuous doesn’t denote anything physical.
Parents and teachers often scold children against slouching and slumping in their chairs. Sitting up straight is considered good, “upright” behavior. The bad kids sit in the back of the classroom, slouch in their chairs, are generally lazy, and get bad grades. The good kids sit in the front of the room, sit up straight, work hard, and get good grades.
The passage you quoted playfully calls people “virtuous” for possessing the qualities of character needed to exert the effort and self-discipline to sit up straight—as if sitting up straight without a backrest were a matter of morality on par with the usual matters where people normally use the term virtuous.