I'm having the exact opposite of this issue on another one of the starters.
I've followed the guide exactly, but it's quite confusing. On Step 4, it says "Now that the pages for your blog post documents are accessible". However, Step 3 makes no indication of how to actually access a particular blog post.
If I implement Step 4, and click the blog post title link from the home page, then the blog post loads. However, if I refresh this blog post, then it 404s. It also 404s, if I paste the URL in and try to load it.
As you can see, this is the exact opposite problem that I described at the end of my last post, where I said that clicking the blog title didn't work, but manually pasting the URL did work.
EDIT: This person appears to have had the exact same issue: 404 when reloading a URL with nextjs + server side rendering