010101 I dont agree with you at many points. First of all, SEO optimizations works and Google also recommend do them. You can start runing the official PageSpeed Insights https://developers.google.com/speed/pagespeed/insights/
Giving an answer to your question:
And, no one has fully addressed my original question. What is Flarum missing and can you prove it’s a big deal?
Now one advice, simply read official Google webmasters blog posts and FAQ, because is the unique trustable source. Blogs and private webpages reinvent the wheel over and over again. You can start here: https://support.google.com/webmasters/answer/7451184
You can continue here (old but still good): http://static.googleusercontent.com/media/www.google.com/en/us/webmasters/docs/search-engine-optimization-starter-guide.pdf
A good structure of headings is critical, because it ranks internally the contents and also makes the contents more friendly and readable. You can use many H1 into same post, you don't need any restriction but... at least the first H1 should be descriptive about the webpage contents.
You have metadescription because you have a SEO extension, Flarum don't offer them by itself.
If you use another language different from english you will understand why some characters can give you issues. My forum is in spanish and i have issues with special chars, i need to use a custom extension to handle them. I know that you think that everyone on the face of the world write in english, but that's not true.
Finally i totally agree that core only should handle basic stuff but i think that we also need an official extension to add better SEO.
