wow, that makes absolutely no sense what-so-ever, given inline CSS can never be render blocking (unless there's @imports in there, but that clearly would be a problem with the inline CSS) and given the deferred CSS is explicitely loaded using JS after DOMContentLoaded.
The only remaining possible culprit would the CSS noscript-fallback; if there's no JS, there is a block of <noscript>-code in <head> which will load the CSS the traditional way (which is render blocking). But as this only happens for browsers that don't have JS, this is not a real-world problem and I would indeed consider this a bug with PSI (which should be easily proved with some webpagetest.org tests).
Now in order to confirm the noscript-block indeed the stumbling block for PSI, you could open up /wp-content/plugins/autoptimize/classes/autoptimizeStyles.css and on line 452 replace
$this->inject_in_html($noScriptCssBlock,array('<title>','before'));with
$this->inject_in_html($noScriptCssBlock,array('</body>','before'));to load the noscript-block at the end of the HTML? Would be great if you could try that on one of your customers' sites.
curious to where this will lead us :-)
frank