I think Thecus may have enabled DLNA on PLEX by default. So plex is the default DLNA media server.
With any DLNA media server, you have to select which shares to serve up. This is a manual operation on all the NAS unit's I've used. It's possible for the DLNA to scan your entire NAS, but that would be a security and privacy network to serve up everything on your network. It should alway be a manual designation of the share.
So if you didn't do this anywhere other than Plex, then PLEX is your DLNA server.
If you didn't log into a samba share with your NAS crediential on your TV, then it's DLNA you're access, not SAMBA. Unless you have an open share on your NAS that doesn't require credentials.
If all of the above is accurate, then there will be some videos PLEX will struggle to transcode or won't allow you to play. However, your NAS seems to have the same processor as the Synology DS415Play. If Thecus managed to get plex to hardware transcode with your atom chip. Then it may be able to transcode things fine, I'm not sure.