The thing is, there was evidence it wasn't working properly. The court rule doesn't change anything.
In the middle of the prosecutions they even settled with some post masters who were able to independently find that the Horizon system worked incorrectly and then they continued to go after other post masters who hadn't figured it out.
The Post Office Scandal [1] is not some story about the dangers of computers being wrong. It's a story about how covering up evidence can allow you to gain a wrongful conviction.
[1]: https://en.wikipedia.org/wiki/British_Post_Office_scandal
10
3