Thanks for reporting this. ReadyNAS OS6 is a customized version of Debian 8 (Jessie), so my article seems to be incorrect about this. Changed it in the howto article.
I don’t know if this was a wrong Copy/Paste action, or that I actually installed the Debian 7 version.
Does anybody know how to verify which Mono version is installed on my ReadyNAS (Wheezy/Jessie)?