Yeah, I am at a loss at this point. When you run the repolist command I am not seeing it show the EPEL repository, yet when you try to install it it says you already have it.
This isn't really a NodeBB problem though, so we can't really blame the NodeBB script for it. It's almost like Yum itself is messed up. You might be able to get extra help from the CentOS community though, or if someone else here is more of a CentOS expert?
I just did an install of NodeBB 0.8.2 and it installed nodebb-plugin-emailer-mandrill 0.3.1 with it which is only compatible with NodeBB 0.9.^. In addition to the declared version problem I was also getting a socket.io/helpers related error on NodeBB startup until I removed the mandrill plugin.
I worked around this problem by removing mandrill and then installing it like this:
npm install email@example.com
I poked around to see how nodebb installs nodebb-plugin-emailer-mandrill and it appears that this plugin is installed in a special way? It is not mentioned in the package.json requirements for nodebb.