It means precisely what it means: F=dp/dt is invalid if applied to an open system; nothing more, and nothing less. You can't go around claiming physical laws are invalid just because you've found ways of abusing them. The only person making concessions to keep their theory afloat here is you, when you're not flat out ignoring the refutations of others. Your site still advertises your twin gendanken, the de Broglie formula, and gyroscopes as disproofs of relativity when you should know perfectly well by now that they don't show anything. Physical theories come accompanied by all kinds of subtle points, assumptions, and pitfalls, exploited by highshool teachers torturing their students with apparent paradoxes and creative professionals proposing convincing plans for perpetual motion machines. The models themselves are consistent, but they often don't appear that way to someone who isn't familiar with them. Why do you think everyone's been urging you to seriously study some physics sometime?