Whoever downvoted my earlier comment (or this one), please explain your downvote.

I think my comment was on-point, truthful...

If you're claiming that you claiming these attributes justifies your post, I note it's circular reasoning. Otherwise, onto the next:

...pithy...

Even in conjunction with the other attributes you list, I'm not sure that pith is even close to being a good thing more often than not. See ciphergoth's post on never being sarcastic

...and not overly rude...

It took me up to now to figure out a plausible (though not necessarily probable), non-insulting interpretation of your comment. Originally it came across as you calling the original poster laughable and naive, and belittling them for being an unsuccessful campaigner. I also originally thought you were laughing at their advocacy of immortality because you are against immortality, but I now think that that might have been me misinterpreting to an extent you couldn't have reasonably avoided.

I genuinely think the post is hilarious, because it shows so many cognitive biases in service of "rationalism."

This is a justification for your comment being truthful, not for it being useful.

The poster claims he wants to reduce X-risk. But his proposed solution is to stand in the street with placards saying "Stop Existential Risks!" And then magically a solution appears, because of "awareness." What would we say about, for example, a malaria charity that used such a tactic?

This comes across to me as you pattern-matching the original poster to the Clueless Activist stereotype and then being discharitable to them retroactively because of that pattern match. Omitting the details of how street activism and awareness-raising causes good outcomes is not the same as there not being any mechanism by which it would work. This feels like it should be immediately obvious if you were trying at all to be empathetic or trying to identify non-crazy interpretations of the original post.

What would we say about, for example, a malaria charity that used such a tactic?

Effective Altruists wouldn't respond like you did to someone suggesting street activism just because it pattern-matches to stereotypical clueless non-effective altruism. EA's don't belittle conventional interventions.

More to the point your third bullet point doesn't constitute a valid argument for making your comment. Even if the original poster is proposing a magical non-effective intervention, you haven't shown why this is significant evidence in favour of your actual comment, rather than just for not taking their proposal too seriously. This argumentative omission seems to be a recurring theme to me when I question someone who belittles a new or inexperienced user; it's easy to mock a new user or make an inexperienced user seem silly, and get upvotes because that rhetorical sleight of hand makes it look like you've actually justified your specific response to them, when all you've done is justify general skepticism of their suggestion.

I seem to recall that policy debates shouldn't appear one-sided. Yet all his slogans are ridiculous. Consider, for example, "Prevent Global Catastrophe!" Do you think that people who don't take existential risks are in favour of global catastrophe? What does it even mean to say there is a 50% chance of a global catastrophe?

As above.

Perhaps the funniest part is that the poster has already organised street actions for immortality. Presumably, he must believe that those made great strides to solving the problem of immortality(!!!), which is why he's now using the same tactics to tackle existential risk more generally...

Unless you have significant specific knowledge of the effectiveness of street action, this collapses down to 'My prior belief, pending further information, is that street action is ineffective.' Which conspicuously isn't a justification (beyond being weak heuristic evidence) for your actual comment rather than general skepticism.

But in another way, his street actions for immortality were presumably successful, because they made the participants (at Burning Man, no less!) feel good about themselves, and superior to the rest of the common flock. So the second part of my comment was a double-edged sword.

Insomuch as your comment was self-effacing and potentially supportive, you did not communicate that clearly.

I could go on. Ultimately, if you make a ridiculous post, you can't expect people not to laugh.

This is a linguistically Clever argument that wins rhetorical points but is extremely non-obvious, particularly in the context of Less Wrong. Again this fails to specifically justify your comment rather than having the reaction of laughing at the post (but not necessarily commenting to that effect).

With all these points, I can make lots of guessesd at what the filled-out form of your argument would be, but I have lots of uncertainty over interpretations and it would be a lot more efficient if you spelled out your arguments.