Actually, it is not my theory, I get that from the whitepaper published by Sergey Brin and Larry Page, founders of Google.
The Anatomy of a Search Engine
Again, you are underestimating the granularity of Google’s indexing. A single web document may discuss multiple topics and Google is very good at indexing that same document for each keyword that may be relevant.
Additionally, the blog post you use in your example has many backlinks, including a couple generated by your own post on this thread, that uses the “spammers” keyword in the URL, anchor text, page titles or text. These are all signals that increase the relevancy score for that keyword.