But honestly the vast majority of the knowledge transfer is happening through more legitimate means. Even the famous Stanford machine learning professor Andrew Ng got poached by Baidu.
I would be concerned about this concept being widened beyond nuclear energy research.
as an example computer ubiquity and internet connectivity allow 4th/5th gen warfar to exist. If encryption or miniturization, or transmission media were considered subject to born secret doctrine that could make things very difficult and somewhat partisan.
Legally, until someone properly tests that in court, then it is not even really law yet. Also, as a legal conceit, it doesn't continue to apply to people after they emigrate from the US.
I feel also, that you can't really go around telling people that they are banned from emigrating due to what they know, unless you also have a labyrinth containing a minotaur, fed with captured foreign citizens.
Although, I have to say, it would not greatly surprise me by this point to learn that this was in fact the case.
when a western scientist goes to work in China, the nutters are angry because of "loss"
if an easter scientist comes to work in US, the nutters are angry because of "loss"
it's just in-group out-group identity politics bullshit...
just like how in times of slavery interracial marriages were a big no-no because it blurs the agreed-upon line of class separation ("which class does the mixed child belong in?" the biological expression of a mixed child [that we are fundamentally the same] contradicts the ideological expression [that we are fundamentally different]) so today any form of mingling is viewed as a threat to warmongers (who want to maintain a simple thus virally contagious worldview)...