The doctor isn’t making any claims about some factor that is sufficient, but not necessary, for an effect. Rather, the argument claims that slipped or bulging disks are not sufficient to cause serious back pain.

The doctor concludes that, since bulging or slipped disks are not sufficient to produce serious back pain, they never cause serious back pain. This ignores that they could still be partly responsible for some instances of serious back pain, and thus still “causes.”

The doctor doesn’t make any claims about an effect that occurs in the absence of some phenomenon. Rather, the doctor discusses how the phenomenon of a slipped or bulging disk is not always followed by the effect of back pain.

The doctor’s argument doesn’t rely on applying sample data to the entire population. The fact that half the sample is observed to have slipped or bulging disks without serious back pain is enough to say that these conditions aren’t always sufficient to cause serious back pain.

How likely bulging or slipped disks (the factor) are to accompany serious back pain (the effect) isn’t relevant to the argument. The doctor isn’t concerned with how often these two things co-occur, but with their causal relationship.