The HPA covers additional stuff like global climatology, human performance and limitations etc. It is nothing to do with the difference between pistons and Turbines although it does cover an element of turbine mechanics.
It is fairly simple course that can be covered in a couple of days.
A turbine may be easier to manage in terms if wiggling the levers, but it has many more issued associated with it. Our turbines are torque limited in certain temperatures and ITT limited in others. Understanding the workings of a turbine before starting a type rating course is a bonus.