So, reading the linked security info, it says: "Each user's iMessage connection runs in its own private space [emphasis mine] that's never shared with anyone else. When you disconnect or delete your account, that space is torn down completely, and nothing of yours carries over to anyone else" and "Your ongoing iMessage session is managed by Apple's own Messages software, the same way it works on a personal Mac." That reads like a shared server to me—spinning up a Mac VM (or, more likely, a Docker-like instance) per user. You're adding a new "device" to your Apple account to pull this off, but you have to rely on their assurances that their staff can't access those instances.
I get why this might be attractive to some, but this app isn't the answer.