I doubt that any of these rules are things the author considers to be proprietary opinions. The majority of them are common guidelines that are echoed by any competent graphic or UI designer. Many things like line length and leading have been extensively studied. Human factors researchers take things like readability/legibility seriously. A lot of science goes into signage or other functional uses of text.

While the presentation may have struck you as pompous, as far as I can tell, the author is just trying to save people time of researching these things and make it more likely that people won't keep building terrible websites which are hard to read or use.

There are a few things like using slightly off-black or off-white that can be controversial, but overall this reads to me like a great place to start if you don't know any better.

These are a set of extremely good ‘rules of thumb’. General engineering is full of them. One rule not mentioned though is this; “Break any of these rules sooner than do anything outright barbarous”, to paraphrase Orwell. However, to safely break these rules, it helps to know and understand them in the first place.