Your question ASSUMES that STOP buttons are wired to some type of contact. I say that a better assumption is that STOP buttons usually OPERATE a normally closed contact. The STOP button is not wired to any contact, but the contact is an integral part of the button and is operated by the button - no wiring required here! The wiring is in the next part, from the STOP button contact to the PLC Input terminals.
Where you made the wrong assumption is: that you wire this Normally Closed Stop button to a PLC input, and then inside the PLC program, you would use a NORMALLY OPEN contact to examine the Normally Closed Stop button input.
You're right. A poorly worded sentence on my part. Mea culpa