The investigators looking into this have already said they haven't found a problem with the batteries.. and they are turning their attention to the electrical system.
You're missing a key point: they are turning their attention to the electrical system that controls how the batteries charge and discharge. That means that, even if the batteries themselves have no design problems per se, their failure modes under an overcharge condition are highly relevant. Which is exactly what Musk and Prof. Sadoway are critiquing: they're not saying the batteries were manufactured badly or designed badly for normal operation; they're saying the batteries were not properly designed to cope with the failure mode that the electrical control system is suspected of inducing.
You're missing a key point: they are turning their attention to the electrical system that controls how the batteries charge and discharge. That means that, even if the batteries themselves have no design problems per se, their failure modes under an overcharge condition are highly relevant. Which is exactly what Musk and Prof. Sadoway are critiquing: they're not saying the batteries were manufactured badly or designed badly for normal operation; they're saying the batteries were not properly designed to cope with the failure mode that the electrical control system is suspected of inducing.