OK, first off, it's generally a good idea to PM the moderator who warned you, or PM an admin if you want a warning revoked.

Second, you're going to get nowhere calling the Staff garbage most of the time, and complaining about it.

Third, I must have misunderstood your post, since a post is a single message from one member. For example, this is a post. The message from Loric above this post is a post. A topic, however, is an entire discussion: for example, this topic is title "wtf," and it is about your warning. So saying "everybody in this post is an idiot" is the equivalent of wearing a shirt that says "everybody who wears this shirt is an idiot." Hope that clarifies.

Now, to answer your questions one by one:

What does that even mean?
The "topic at hand" means the topic of the conversation. In that thread, it was who your favorite rapper was.

Even if I did, is calling myself an idiot against the rules?
Not necessarily. However, when you suddenly call yourself an idiot in a topic which is about rapping is off-topic, and really makes no sense, and is useless because it has nothing to do with... Anything at all in that topic. Therefore, it is spamming.

Are the mods going to hand out infractions for every single argument on this board?
Not at all! The Staff encourages intelligent debates and conversations. I now understand that you weren't calling yourself an idiot, but rather meant that "everybody in this topic is an idiot." (Still, though, you are still calling yourself an idiot because you are also a particapent in a topic when you post a message.) This is highly offensive! It isn't even an argument, it's an insult, and insulting someone because you disagree with them is against the rules. Next time, I suggest that you articulate an argument with points and logic rather than just insulting them.

Hope this answers your question!

I would change my infraction from spamming to flaming/trolling if I could, but since they're practically the same punishment, and you broke the latter, it would be unnecessary.