If "both involve deaths with laughter" is the only context, I'd agree that that's a shoehorn.
Regarding ACT's scope/scale, I think the trope should exclude in-universe analogs since that leans more towards Genius Bonus, which is more of a free-for-all—the definition isn't too broad, but gives more leeway for things that don't really have any relevance beyond enhancing certain viewers' experience, like matching abstract concepts together based on something they remember from an old college-level physics assignment. Most examples of the former I've seen involve direct depictions of something irl, and I don't remember a time when they weren't namedropped (the Trope Namer obviously had to have been), so I think that set a precedent of there being a lot of humor in things that sound like they're made up, not just look like they are.
Your goateed philistine is sashaying towards us. | 🧱Aluminum Christmas Trees is when the audience is confused by realistic things and assume it is Artistic License when it is actually mostly true, the confusion comes from a matter of presentation or Values Dissonance that makes them assume it is false.
The example given sounds more like Art Imitates Life as it doesn't specify the belief it was made up for the movie. Not to mention Jokers laughing gas poison is a long-standing comic trope, so it didn't originate from the movie.
Comics are just words and pictures. You can do anything with words and pictures.

I tried to look for an official Aluminum Christmas Trees cleanup thread, but couldn't find anything, so I'll ask here - exactly how close does a fictional element have to be to a real element to qualify? I'm specifically asking about a YMMV item on Batman (1989), which compares the Joker's laced hygiene brand to kuru (an encephalopathic prion disease) because they both cause deaths that involve laughter. I..for one, I don't know that this would have been written as an intentional reference to kuru (since this would have been pretty obscure before mad cow), and they do not work similarly, but is this actually a valid example of ACT?