I can totally see that happening under the circumstances. Sometimes criminals get cocky and arrogant and run their mouths when you would not expect them to utter a word.
That is a rather severe distortion of what Garrison claimed.
Read Garrison's books "Heritage of Stone" or his earlier work "On the Trail of the Assassins" for his fuller description of who he said was behind the assassination. Briefly, Garrison claimed in both works that major elements and figures of and inside the government, e.g., CIA, Pentagon, LBJ, Hoover along with the Dallas Police Department murdered JFK (Ruby shot Oswald to hide his (Ruby's) role in the assassination). And that the plot (somehow) originated or had its origins in the Shaw/Oswald/Ferrie meeting; although this meeting essentially disappears as he gets deeper and deeper into explaining who carried out the assassination. It's a bewildering story.
Why did they kill Kennedy? Garrison said that JFK was, inter alia, going to end the Vietnam War and dismantle the "warfare state" that secretly ran and runs the country. And it was essentially for those reasons that he was murdered, indeed
had to be murdered. Garrison also claimed that RFK (!!) was covering up for the murder of the president. It's nearly impossible to caricature his claims.
Just read the final chapter in "Heritage of Stone" titled "War Machine." Here are just two quotes that aptly summarize his worldview:
"The assassination [of JFK] reduced the President of the United States to a transient official, a servant of the warfare conglomerate. His assignment is to speak as often as possible about the nation’s desire for peace, while he serves as a business agent in Congress for the military and their hardware manufacturers…
And this: [T]he military and intelligence power elite, which sponsored the assassination and which then initiated the Vietnam escalation, continues to retain covert control of the nation. It is all too apparent that this force in our government believes that violence is the ultimate solution to any problem. This is why the present period is a most dangerous one for America and for the world…."
An online version of "Heritage" can be read here:
https://www.ratical.org/ratville/JFK/HeritageOfStone/index.htmlOliver Stone's movie JFK greatly misrepresents what Garrison claimed in the Shaw trial. In the trial Garrison didn't make the above claims. But later in the above two mentioned books he made this grand conspiracy argument.
As to the first point: No, I can't see people seriously plotting to murder the president - and then doing so (according to Garrison) - while a group of strangers were around and able to listen to their discussion of their plan. If they were just "spit balling" things, yes; but if they were serious, as again Garrison claimed, then I can't.