Ethernet transmission is broadband not tuned like a radio but the same equations as presented in the article hold across most of the bandwidth (where the skin effect holds). Actually, I am not sure whether the article is referring to tuned or broadband communication but in a sense it is irrelevant because practically, a large proportion of the energy transmitted in a modern broadband system is in the skin effect domain. If this wasn't the case, the system would be highly inefficient in the use of available bandwidth.

Because 50 ohms is an universal standard for most radio frequency equipment, not just microwave. It (often) doesn't make sense to invent your own coax, connectors, transformers, and chips. At the end of the day, the selection had its rational back in the days, but it eventually became an agreed convention that everyone just follows.

Because they were using existing BNC connectors and cables.