What they're asking is why Traefik/Caddy and not nginx? Do they offer anything that nginx doesn't? Nginx is pretty tried and true and honestly, in this space, I personally prefer tried and true over new and shiney unless new and shiney offers something I need that old faithful doesn't.
To be fair, I was an Apache hold out for a looong time until I accepted my lord and savior nginx into my heart.
Why are you proxying to your VPS external IP? Caddy listens to your external IP and your arr apps listen to ideally localhost, you should be reverse proxying to localhost. Your arr apps need to be told to only bind to localhost as well.