"secret authoritarian “plot” to preserve the ability to spread antidemocratic conspiracy theories" - Applebaum is an American citizen so she should already know this but it's not an "ability", it's a right we all share. Say what you like, read what you want.
If she is right in her assessment despite the wording, and these rights are something like a security vulnerability for societies, limiting them is not the only option. You can as well decide to use compensating controls (education, minimum age for social media, more bot control, limiting foreign agents access - not endorsing any of this, just some examples I heard elsewhere. This would thwart the ability without changing the rights it seems, which shows they are something different), or accept the risk tied to them as lesser than the gains.
using national security as though were intrinsically a bad word is a red flag in itself. You could restate this comment as “the American government is concerned about negative impacts of these developments and so we should ignore these efforts to subvert democracy, and so doing, accomplish the goals of the propagandists.”
Your accusation, which is straight up ad hominem, reveals more about you than Applebaum. Applebaum is not only respected in her fields, but her logic tracks and her positions well reasoned and defensible. You may not agree with her, but that doesn't make her a propagandist.
Applebaum checks the standard neocon boxes and been a member of standard nat sec talking points laundering cutouts - Carnegie, WaPo, The Atlantic, etc.
Some of us have not forgotten those who cheered on the Iraq war based on lies that led to a million people dead, displaced or maimed. Applebaum cheered on every single war in the name of democracy. So yes, her grandstanding about democracy and propaganda does not pass muster among well informed people.