Salt changes the boiling point of water. If in certain conditions pure water will boil at 100 degrees, slated water will boil at a slightly higher temperature. If the pasta will cook in 8 minutes at 100 degrees, it will cook in slightly less time in the higher temperature the salted water reach.

However, you should add the salt to the water after they already boiled, and then let them boil again. If you add the salt to cold water it will take longer to boil and the whole process will be slower. I guess this is the reason for the contradiction you asked about.