I think the whole point of the post is to poke fun at how Microsoft is cheating the benchmark and how it was caught. The part on the definition of Angles and that the "optimizer" should throw an exception it doesn't (most probably because it's not optimizing anything) is hilarious.
What are you talking about? That code is getting optimized away into a no-op when it shouldn't be, which is why no exception is thrown. This means that
a. the optimization is definitely happening
b. it is unsound
I also think the "for beginners" in the title is somewhat directed towards whoever implemented the "optimizer".
That isn't unsoundness, that's incompleteness. An optimization being incomplete means that something that could be optimized isn't -- one being unsound means that something that shouldn't be optimized is. These terms come from mathematical logic.
Being incomplete means the code you generate is not as fast as possible. Being unsound means the code you generate may be wrong. We don't normally call an optimizer "broken" if it does not optimize something even though we can recognize the optimization could be applied - no optimizer does every conceivable thing. But generated code that is not semantically equivalent to what was given violates correctness.
The only reason to trust this code to do other optimizations well is all the evidence it's targeted at one specific benchmark.