One reason we decided to keep this clause in the Caddy EULA (which I should clarify here only applies to official binaries, not the open source, Apache-built binaries you can make yourself) is because we found out that very few people are expert enough to benchmark correctly. I've read a dozen Caddy benchmarks, for example, that turned out to be based on false assumptions or had hidden factors or were simply not reproducible (and not just by me).
Benchmarking requires expertise that, it turns out, very few people have. I don't think I even have enough skills to do it correctly and meaningfully.
Also, web servers are complex enough (in terms of both configuration and all the layers involved with networking stacks) that one correct benchmark is not generally useful to the next person.
Spreading wrong performance information can hurt a business. It's not that there's anything to hide or any desire to take away your freedom -- and I would normally be one to assume the worst from any large company -- it's just business: they don't want the risk of bad PR based on a possibly false premise, especially when that information tends to only create negative hype rather than actually being useful.
Anyway, this link doesn't seem like news. Just usual HN hype.