The higher the temperature the faster connective tissues break down. To break down the same amount of that connective tissue at a lower temp like 165 you need to leave it in for many more hours, thus the 18 to 24 hour cook. What you are getting with the sous vide is the same breakdown of connective tissues (thus tender meat!) but without having the meat itself cooked to that “high” temperature. I defer to chefnoir or Ember to provide a more complete explanation, but I think this is the answer in brief.
I’m looking forward to hearing how your pork butt comes out!