>Trying to prevent China from obtaining tech is futile.
In general, you are correct.
However, there is some tech that is so closely guarded that even its existence is only known by a small number of people.
Imagine if a small team working for the military figured out how to create a "targeted" bio-weapon that would kill the target but leave everyone else asymptomatic. Even the existence of such a thing would be so closely held that only the team working on it and maybe a few higher-ups (likely including the President and a few lawmakers who deal with black-ops budgets) would even know it existed.
In this case, "security through obscurity" in combination with all of the other operational security that would go into such a project would likely keep it out of the hands of the Chinese.
Sure, the Chinese may be working on something very similar, but very tight security means they won't get there any faster than if they were the only country/organization in the world working in this area.
By the way, this particular "small team" example is entirely hypothetical (I hope). But substitute "nuclear weapons research in the early 1940s" and the concept is similar. A major difference is that nuclear weapons research involved tens of thousands of people instead of a "small team." However, only a small portion knew what the research was really about or had the skill set to make a good guess.