Hate to spoil all the fun, but < optgroup > existed since HTML 4... Half of those tags aren't things an average developer would "not know"
That's acknowledged in the article:
Some of these tags are either brand new in HTML5 or have been repurposed from the HTML4 specification.
I'd known most of these prior to reading the article, but it's a useful reference for some real world examples of when to use some of the lesser utilized tags. The W3C HTML5 spec can be a bit lackluster at times :)