I think you might be conflating two different statements. He said the damage to the taillight on KR's car is similar to the damage in their tests when they fired a glass at the taillight at 37mph. He then says the injuries on JO's arm are not consistent with being hit by a car.
That's great for the defense. He's disputing the CW's theory AND providing a potential other explanation for the broken taillight and the glass evidence - the defense doesn't need to prove anything, just provide other, logical scenarios to explain XYZ
***I will add the caveat that I have no clue if 37mph is a reasonable speed for someone to throw a glass at. I have absolutely zero frame of reference lol. If it's not, that still helps the defense that a glass at a fairly higher speed than KR driving is what caused that damage.