I've worked on software for machine control.  If a sensor reports incorrect values, that \*is\* a failure, you may or may not know it but it's a failure.  The term "off scale low" or "off scale high" can be used in some cases.  In a correct test suite, it would have tested what happened if said sensor report nothing in addition to high and low values.  As I said, not an aeronautical engineer but have worked on a variety of machines and in some cases, a chamber had to heated up to a certain temperature.  We would test, what happened if it was too high, too low, no report whatsoever, taking too long to reach a temperature, even if too short.  If I say, "go to 160C" and it immediately says it's at 160C, something is probably wrong.

So, you've answered your own question, "What happens if it reports a too high/low value?"  That's the test you do.  You can't say if the value is correct but you can test to see what happens if it says you're at +12 degrees when you aren't.

You poke and prod the system.  If it says, "How many people are there?"  You put in -7.  You put in 7, hit the cancel button, then put in -12.  If it asks a temperature, you give it an impossible temperature of -1000c, you give it absolute zero and see if it acts weird.  That's what testing is about.  You have both rigorous test suites that you run every day and then you also allow a tester to play and see what they find.

So yes, the second AOA sensor wasn't used but then you test under that case.  I'm not saying no testing was done but something broke down somewhere and now they have to find it.