I had a wound that became infected last year. My doctor told me that a wound is like an open gate for bacteria. Bacteria don't only enter a wound easily, but they also thrive in it because it has a suitable temperature and environment and it's full of nutrients for them. When bacteria enter, neutrophils get activated and create pus like the article described.
By the way, pus is not just a sign of infection, it's also a sign of inflammation. One of the duties of neutrophils is to cause inflammation.