I would think it's accurate to state that the system never had a failure in flight if it never had a failure in flight, despite how many times it flew.

If you were to compare it to something else, then how many times it flew becomes important. For example, you can't necessarily say that a 100% success rate over 5 attempts is better than a 95% success rate over 100 attempts.

Right. I think I remember Digital Apollo quoting an engineer who estimated, based on their extensive ground testing, that the AGC had a MTBF of about 50,000 hours. That's not particularly good, but considering all the Apollo missions spent only about 1000 hours in space total, it was apparently good enough.