But diverging into a discussion of elemental physics is fundamentally missing the practical difference: Customs and Border Patrol can seize a kilo of heroin. Can they, in actual fact, seize the aether?
Why do you think I mentioned electrons specifically and not copper wires, optical fibers, etc.?
What do you think radio waves transmit through the vacuum of space where there are no electrons? It's energy, not matter.
But that still isn't the point. The point is that the United States does not in fact and could not feasibly in practice block all radio transmissions from outside the United States from reaching people inside it. The First Amendment may prohibit them from doing so even if it was technically feasible, which it also isn't.
The parent comment is clearly referring to a terrestrial border, on Earth.
And in any case humans currently only possess the technology to receive radio waves, even in a hard vacuum, via electrons. So 'Data' that matters to humans is always physical, with tangible non-zero mass, 100% of the time in 100% of all conceivable situations circa 2023.
But most relevantly, there is no practically feasible barrier you can use to keep them out of your country.
Also, what does 'practically feasible' have to do with the parent comment?
A lot of things that are not practically feasible are nonetheless mandated in some form or another, and are still punished for if a transgression is detected via happenstance, there are many examples throughout history.
Yes it does, at least the type that humans can access. Electrons have mass, momentum, spin, etc...