To clarify, both script-injected and regular <script> tags block on CSSOM: there is no difference there. Only the "async" attribute on script (i.e. <script async>) removes the dependency on CSSOM... which is why you should use it.

As for why the browser doesn't download the script-injected scripts: they're hidden inside of the inline script blocks. In order to figure out what you intend to do inside of that block, the browser would have to execute the JavaScript. The behavior that you want (download wait for CSSOM) is exactly what you get when you use regular blocking tags - those are discovered by the preloader and requests are dispatched. That's what the first and second diagram in the blog post illustrate.