Roon authenticates your user credentials and uses an encrypted transport. No unintended users can access your Roon.
If Roon is not running, then no one listens on the port that is forwarded, so nothing external can access anything inside your network. It’s like a phone number that never answers.
If somehow you already had rogue/malicious software on your network, it could access your network freely and it doesn’t need to open ports at all. It can tunnel traffic from the outside to the inside freely. UPnP or not, it’s game over if you have malicious software inside the network.
Those claiming that the UPnP setting make your network insecure are clearly not familiar with how malicious software works. If some rogue software could configure your router to open ports, it can also do that by opening an outbound tunnel. In fact, it’s probably more reliable to open the tunnel than it is to mess around with the router variations and UPnP/NATPMP/PCP support. But forget about tunnels or ports, it can just do the nefarious actions itself. At that point, its all up to local OS security.