Can we just go ahead and call this straight up fraud? If you advertise a paid course on C# and don't have the first clue about the language, that can't be assumed to be in good faith.
Benchmarking is hard. If not an art then at least a distinct skill. It goes beyond just using Bnechmark.Net. It's a good first step but far from enough. Oftentimes one need to make sure a compiler does not optimize away some benchmark paths, e.g. by using volatile field accesses. Especially if you want to avoid overheads and measure only certain things. But here it was so obvious it could be meme-tagged #YouHadOneJob.
Just because you put a name on a plain wrong idea doesn't make it true.
> Benchmarking is hard.
There were no benchmarks since the author didn't even understand how to write simple C#. A subtly flawed benchmark would be a whole different story.
Author has made a career out of providing teaching materials, but hasn't worked to solve actual problems (e.g. in industry) for so long that their skills have degraded. One possible explanation.