What is the difference between shoulder impingement and shoulder tendinopathy?

Exactly, it is important to understand the definition of tendinopathy.
Tendiopathy-is clinically defined as a degenerative process of a tendon or chronic tendonitis.
Typically of the bicep tendon, supraspinatus and patellar tendon.

Shoulder impingement is not a type of tendinopathy. As it does not create inlfammation because of the affects anatomically. There are two types of shoulder impingement-primary and secondary.
1-Primary impingement is pain that is caused in the anterior aspect of the shoulder due to a enlarged acromioclavicular joint or due the mechanical crowding that occurs with raising the arm.
Patients will have pain between 60-120 degrees typically because as the arm is raised instead of normally translating down, the humerus migrates superiorly in the to AC joint. This has been postulated that this is because of a tight capsule.

In my 12 years experience, I have found men and women typically due have a tight posterior capsule.

2-Secondary impingement is due to weak scapulothoracic muscles. Where the scapula does not properly glide on the thoracic when the arm is raised to the side and instead of normal upward and outward translation of the of the scapula, patients will demonstrate elevation or decreased outward upward rotation. This is due to weakness in serratus anterior(which protracts and upwardly rotates) and biomechanically with the upper and lower trapezius causes upward and out rotation of the scapula.

This is why it is vital when someone who has had shoulder impingement syndrome, to strengthen the serratus anterior and low trapezius biomechanically and based on science.

PTCS will be having short Utube videos in the future discussing this and showing this in the near future on here.