Why unofficial installer on flathub?

I want to start with an appreciate for Bitwarden's work, it's amazing. But I have a question. Bitwarden support deb/rpm, appimage and snap installer, but why you let third party publisher manage the flatpak installer on flathub? I have no idea how does appimage, snap and flatpak work, maybe the answer is kind of easy and that's a stupid question, but I can't understand this.