I dont think you have ever seen a Sodastream bottle - It screws in to a fitting yes, but it also has a release valve built in to the tank. In Stealth the valve in the bottle is the firing mechanism and everything past the valve is vented and low pressure. Fitting a second valve after the tank makes the system LESS safe as there is now a section of pressurised tube between the tank and the valve.

In this bot, there is also a dump valve just before the ram that is open at all times other than during the firing cycle; it opens to retact the ram and stays open until the next firing. I believe it failsafes open by spring tension.