Pressure isn't the issue, expansion is the issue, pressure is your only reliable measurable quantity that would calculate the mass of propane to safely be decanted into a can.
You cannot 100% fill a colemans bottle as it needs expansion space. See the combined gas law to understand the relationships.
This is also why GHK recommend you only fill for 6 seconds.
That Relief valve is designed to be bespoke to colemans specific mass of gas and is designed to vent as their ceiling is broken, but it's only an emergency vent for high pressure, it's not for regulating safe internal pressures. If that vent goes you are filling the atmosphere with oxidised propane, which is the dangerous bit.
Liquid propane is safe as demonstrated in the videos, it's expanded vaporised propane that's the dangerous part.
That's why LPG cars are safe, they have the failsafes built into the pump and tanks, they are not simply metal receivers.
Pistol and m4 mags use such a small amount of propane it's negligible, 3g max in most instances, and it's burn off time is sub half a second, so any ignition is very short lived.
But we are talking much much greater masses and volumes of vaporisation in a 400g colemans tin. For the sake of £8.