To elaborate on what @Delta_G said already, this is incorrect. When you click that button, no category is selected:
If the user attempts to publish the topic without explicitly selecting a category from that "category" menu, the forum software will block the publishing with a "An error occurred: Category can't be blank" dialog:
So the only way someone can publish a topic in the #using-arduino:uncategorized category is by selecting that category from the menu:
We created the #using-arduino:uncategorized category because previously when people arbitrarily chose the first category on the list, the miscategorized topics ended up in the #using-arduino:ide-1-x category where it was less likely that the moderators would spot them since they were mixed in there with topics that are actually about Arduino IDE 1.x.