This is a common misapprehension. It doesn't work that way. Here's a five minute explanation.
Usenet consists of a huge number of news servers mainly at ISPs but also at universities and in companies, each of which carries a subset, usually a large subset, of the available news groups. When you read news you read it by connecting your news client software to a news server to which you have access and the client software pulls over any articles it's not seen before for you to read.
When you post a news article you are posting it to the news server to which you connect, and only to that server. So it's then immediately available to read for people who also use that server to read news, but not to anyone else. Some time later (usually not much later these days) your news server peers with one or more other news servers and your article is passed on to those servers, and they do the same with other servers until the article has propagated around the world.
That's the easy part and, on the whole, it works pretty well these days: most articles do end up on most servers.
Points to note at this stage:
There are some local restrictions however. The biggest one is that the owners of some news servers restrict the sort of postings which are allowed. So for example many news servers won't accept a posting with an attachment to any group other than one with the word “binaries” in the group name. So a posting to cam.misc with an attachment might get accepted by the original server but won't propagate to every server worldwide.
Now what happens if you screw up and want to delete an article? In that case you can cancel the article. Your news client normally has an option to help you to do that. A cancel request is treated much like an article. First it's sent to your local news server, which then removes the original article from its database and then the cancel request slowly propagates worldwide in the same way the original article did.
But now the “fun” begins. The problems is that the cancel protocol is insecure, so it's very easy to fake a cancel request if you know what you're doing so someone else can cancel your posting if it annoys them.
As a result of this most news servers no longer accept cancel requests so if you do cancel an article yourself it will not be cancelled everywhere.