It wasn't a "software glitch" that caused anything. Yes the software sucked, but lots of software sucks. It was human decisions that caused this, by using software they knew (or could and should have known) was not fit for purpose. And by ignoring people who told them it was not fit for purpose after it was deployed. And the government not stepping in. And National Federation of Sub-postmasters. &c &c &c. All of this is well documented.
That the initial mistakes were made was kind of ridiculous for lots of reasons, but okay, mistakes happen. But that it took 20 fucking years to correct while the broken software continued to be used is just indescribable, and absolutely not the fault of any software but the result of human choices. That's the real problem; in an alternative universe people realized Horizon was a piece of crap shortly after being deployed, the mistakes were corrected, and they fixed it or stopped using it.
Humans caused the misery. Not software. The software bit is almost a minor detail IMO. By framing it as "a software glitch" people will focus on software to "fix" things, but that's not where things need to be fixed.