How to send multiple emails after creating a node
As noted on the Reddit thread, this event will not work as described in this blog unless you are using PHP FPM. For most hosting providers this is the default setup. So, if you are using Apache with mod_php you are out of luck, it seems. 

Are you hosting with Apache and mod_php? You'll need Apache and use PHP FPM with some settings changed. Or use nginx.

Sounds like the problem is due to your server not streaming the output. Per the Symfony docs


  Internally, the HttpKernel makes use of the fastcgi_finish_request PHP function. This means that at the moment, only the PHP FPM server API is able to send a response to the client while the server's PHP process still performs some tasks. With all other server APIs, listeners to kernel.terminate are still executed, but the response is not sent to the client until they are all completed.


https://symfony.com/doc/current/components/http_kernel.html#the-kernel-terminate-event