How much optimization do you expect a loop with a simple addition to have? It doesn't seem a representative workload for an optimizer.
Most likely function call overhead (and in the slower cases, interpreter overhead) is probably what is being measured in the best case.
[1] In that it won't be a jump and check style loop.
Quite likely: https://godbolt.org/g/xGkCyA although it really depends on the compiler (gcc is smarter here). He also seems to be benchmarking printing more than anything else (hence the difference between C and C++).
Without versions of the compilers and explained methods of measurement, the results posted on the blog are more anecdotal knowledge than any kind of benchmark.