All the images in the article are showing that the hand with missing fingers is the left hand. Most people are right handed, so missing a left finger for most people wouldn’t hurt the grip strength of a one handed weapon/tool much in the main (right) hand. You could attach something like a shield strapped to your left arm and wouldn’t notice the issue for hunter gathering then. (Shields aren’t just for defense against claws, they can also be a bashing tool.)
Now, I doubt they were cutting off a pinky finger because they were bored on a Saturday night, but there could have been religious/group beliefs involved. Body modification has been around for a very long time, from as simple of scarification, to tattoos and piercings, to removal of body parts (circumcision), so it isn’t out of the realm of possibilities.