Really quickly: The NodeBB folks favor Ubuntu. The CentOS guide you linked is pretty dated, as evidenced by some now defunct links. I did not use that one so cannot speak to it. It may perhaps be more expedient to move what you have to some backup dir and do over following Scott's guide. I know that works. Moreover, Centos 7 default repos bundle really dated packages. Recommend you set up nodejs and nginx repos if not already.
If you have gui access on that box, I would be curious as to what happens if you bypass the proxy and jack straight into NodeBB on port 4567? If not and you want to try this from an external network, I recommend configuring firewall such that only YOUR ip is allowed to hit that port.
If it then works, then something is amiss with nginx config (wh/I do not have time to review just now.). If not, then more likely NodeBB conf issue.