I notice that they didn't say "here's the results of our last independency audit and verification of correctness", which I think would be a fantastic counter-argument... if they had one.
For that kind of product, source code is not actually that valuable in itself; it's the standards compliance, reliability and trustworthiness. Most charitable explanation is that the vendor is clueless about what their value really is, least charitable is that they know exactly how fucked up their code is.
If its a multi million dollar affair like they claim its virtually impossible that every defendant will be able to fund such an affair.
In fact in the case that a disastrous flaw is found it may be advantageous to simply drop the case and hope that past and future defendants wont be able to each afford to press the point.
In the very next paragraph they say:
> The company offered the defense access under tightly controlled conditions outlined in a non-disclosure agreement, which included accepting a $1m liability fine in the event code details leaked. But the defense team objected to the conditions, which they argued would hinder their evaluation and would deter any expert witness from participating.
So it's a concern about IP protection for them.