This is nonsense masquerading as advice. "Add indexes ... but don't add too many" is a perfect example. It's 100% correct ... and also 100% something no one can actually change their actions based on ... which means it's also 100% worthless advice.

An intelligent reader might read that advice, realize they don't really know what indexes are nor how to use them, then do some research and learn what they need so that they can use indexes to their advantage.

It would be a extremely long article it it went into detail on everything.