I'm not sure your argument proves your claim. I think what you've shown is that there exist reasons other than the inability to create perfect boxes to care about the value alignment problem.

We can flip your argument around and apply it to your claim: imagine a world where there was only one team with the ability to make superintelligent AI. I would argue that it'll still be extremely unsafe to build an AI and try to box it. I don't think that this lets me conclude that a lack of boxing ability is the true reason that the value alignment problem is so important.