One pretty obvious piece of evidence for faulty software would have been if there were significantly more such accusations after the introduction of this software than before. I wonder if anyone asked for these statistics.
Of course they can claim anything they want, but it could have weakened their argument considerably. As a defense lawyer I would have asked for those numbers to raise some doubts, not because it's incontrovertible proof.
Clearly they did not have to do these things.
In addition, the Horizon system only offered the accused the "client-side" data, so to speak - they could see what they had inputted and what the system had printed out. They couldn't see any kind of internal transaction data, or what happened to the data once it was inputted. Post Office repeatedly obstructed attempts of the accused, their lawyers, and even the third-party auditor they hired to examine the system for faults to retrieve that data - likely because they knew that handing over that data would immediately show the flaws in the system.
The whole thing just stinks of corruption, to be honest.