The whole issue is that the current administration has determined that they are the sole arbiter of those "rules", and they can detain and attempt to deport green card holders without any due process.
Providing material support to a terrorist organization is where it crosses into criminal territory.
Once you are in, you have the same rights to free speech as any citizen, and the same rights to due process as well. In fact, the only thing they can do to curb your free speech is prove in court that you lied on your application. Which is likely how this case will play out after judges rule on how illegal their attempted action is, but as of yet the government has not provided any evidence of him lying on his application.
Our Constitution and Bill of Rights is framed in terms of recognizing independent-existing natural inalienable rights. Citizenship is a complete red herring. The SC has been writing justifications for blatant infringement of natural rights for decades, even before the current neofascist takeover. Take the Bill of Rights as a list of test cases, run down them, and try to find one that is actually passing.
Reno v. American-Arab Antidiscrimination Committee made it clear that there are no First amendment protections from deportation. This is well established case law for decades now.
https://www.aclu.org/documents/reno-v-adc
This is why talking confidently about things you have absolutely no idea about makes you look foolish.
Pragmatically, yes, Khalil will likely be deported due to having exercised his freedom of speech. This, along with many other current affairs, should be abhorrent to anybody who actually believes in the ideals of individual liberty - regardless of the justifications crafted by the judicial, executive, and fourth estate. And I really don't know what you gain by cheerleading authoritarianism, besides some feeling of smugness of being on the winning team.