The engineers would not have been testifying about sentencing, but about whether the system output was accurate. You yourself have just argued that all accounting systems have flaws, the engineers undoubtedly knew that, and if they testified to the accuracy of the output, then they would be culpable of making material claims that they knew had not been verified, and could be wrong.

> ...but the judge is equally responsible not to have independent experts (plural) verify those claims.

It is very obvious in retrospect that quite a few people should have been more suspicious, but that would do very little to absolve Fujitsu of the high moral and legal responsibility of telling the truth, the whole truth, and nothing but the truth if they were called upon to do so. "You should have checked what I said" is neither a legal nor a moral excuse for perjury.

Update:

"The data used in court has since been proved to be wrong and former Fujitsu members of staff are currently being investigated by the Metropolitan Police for potential perjury in the trials of subpostmasters that were blamed for unexplained losses."

https://www.computerweekly.com/news/252509066/Fujitsu-escape...

To be clear: None of the above in any way absolves the Post Office of its much greater responsibility.