This wouldn't really interact with the existing port forwarding system at all. The point is to not have to forward any ports at all, all traffic to your public IP would automatically be forwarded to you, circumventing the entire port forwarding mechanism. The advantage is that you don't have a limitation on the number of forwarded ports anymore or restrictions on which exact ports are available. You'd have access to the entire range of 65535 ports. This is useful for several scenarios, for example if you have multiple clients that need port forwarding you run out very fast. It's also useful for punching through restricted networks or heavily NATed/CG-NATed networks and get a publicly addressable IP. Useful if I want to e.g. share a file with someone on IRC but we're both behind CG-NAT, or if I want to spin up a http server to show off a demo but the cafe I'm at blocks incoming port 80.
As for the server infrastructure, stateless address translation is less resource intensive than stateful NAT, so the more popular of a feature this is the less the routing overhead on the servers will be.
There's plenty of ways for spammers and other evildoers to do that for free already, they wouln't need an AirVPN subscribtion to get trillions of ipv6 addresses. Which is why with ipv6 nobody blocks on a per-address level, but prefixes.