Do you really believe the Mossad is going to assassinate (or otherwise seriously mess with) someone in the United States for reporting a funky, antisemitic conversation?

Obviously, no one said anything about "assassination" by Mossad. Your attempted use of reductio ad absurdum was in bad faith.

One would likely be more concerned with being accused of "anti-semitism" and having your name google-bombed by a Zionist smear campaign like Canary Mission. [0] Imagine having a potential client or employer finding your name on a list of "anti-semite Israel haters" for doing nothing more that suggesting Palestinians have human rights or that Israel engages in an aggressive espionage campaign against the United States?

[0] https://canarymission.org/

Can you give an example of someone who is on that list for nothing more than suggesting Palestinians have humans rights, or that Israel engages in espionage in the United States? I clicked on a few people at random and they were all there because they advocated killing jews.

here

https://canarymission.org/professor/Jessica_Barnes

I thought that it was pretty self evident that supporting BDS is different than "suggesting Palestinians have human rights or that Israel engages in an aggressive espionage campaign against the United States?", but apparently not everyone has caught on to the fact that words have meanings.

If they believe the person will carry out an attack, why not??

I believe the speculation is that they were on a fishing expedition to dig up political dirt / leverage, not perform some type of dangerous attack.