You would have to remove the comment line.
You actually probably don't want to edit the /etc/dovecot/dovecot.conf directly, because the changes you make there will go away when cPanel is updated (or whatever process that decides to recreate the dovecot.conf file runs). You would actually want to make the changes in /var/cpanel/conf/dovecot/main if you want it remain persistent.
The post I made was mainly directed towards @cPanelLauren
who I assume has a bit more understanding of how these configuration changes work. Modifying the /etc/dovecot/dovecot.conf file directly can be an easy way to test something, and that's basically what I was after in my intention with that post.
Back to the original point of this post, I don't think K9 Mail on Android will work with Dovecot, unless the DHE-RSA-AES128-GCM-SHA256
cipher is enabled. I may be wrong. And this may be a specific K9 Mail issue. It may be that I'm using an older version of K9 Mail (I know they used to release updates more regularly from their website than from the Play Store). This may be an Android issue. I really don't know. I do agree with cPanel that this cipher shouldn't be used... BUT that's the problem here. The correct way of doing something (i.e. not using this cipher) is not what the vast majority of applications are doing (i.e. K9 Mail, Android, etc) so clients and end-users are stuck using an insecure cipher to continue to use their application. And then there's no incentive for applications (like K9 Mail or Android) to ever change their tune and support modern, secure ciphers. And then people, email, and websites get hacked, and we all wonder why.