The comparison of "strength" of urethral sphincters in the different sexes is unverifiable and pointless. Moreover, the discussion on sex differences in the bladder are unrelated to "sex differences in urethral sphincters".
Whoever wrote this initial discussion may be trying to explain the more frequent observed phenomena of urinary incontinence in females as opposed to males. This discussion does not seem appropriate here; this difference is more likely attributable to the shorter urethra in females, as well as the weakening of muscles and nerves in the urogenital region after parturition.