I think it might mean it is difficult to determine whether post or ante mortem, but like you say, not impossible. I cut and pasted below the point in that article.
Do you think that the police questioned the hairdressers because tests might still have been inconclusive on the Tuesday after her body was found, (ie the day they went back to question them) and they wanted to clarify whether any injuries were already there, whilst they waited for more accurate confirmation from the examiner?
Corpses in water always lie with the face down and with the head hanging. Buffeting in the
water commonly produces post-mortem head injuries, which may be difficult to distinguish
from injuries sustained during life. The presence of bleeding usually distinguishes ante-mortem
from post-mortem injuries. However, the head down position of a floating corpse causes passive
congestion of the head with blood, so that post- mortem injuries tend to bleed, creating the
diagnostic confusion.