If I'm reading the article correctly, the headline claim is about a bug that was discovered in the original Horizon system right as it was about to be replaced

If I'm reading the article correctly, the headline claim is about a bug that was discovered in the original Horizon system right as it was about to be replaced with a new system, Horizon Online, at which point the bug fix would be completely useless. Other things like bugs in audit reports that continued to matter and affected the prosecution of subpostmasters were officially priorities, they just did a terrible job of handling them.