Ligaments are static structures designed to support joints by holding to bones together. They stretch a little when joints move. Tendons connect muscles to bones. They contract and expand while helping to move one bone to another.
In this case the ulnar collateral ligament holds the humerus and ulna together and keeps the elbow stable during the throwing motion.
The flexor tendon refers to the wrist flexor muscles. While their primary function is to bend the wrist. However they originate near the elbow and cross the joint right over the UCL. The tendon acts as a secondary support to the elbow joint.
The issue is that a pitcher can strain the wrist flexor muscles while flexing the wrist while throwing. This joint structures maintains its stability in this case.
However the muscle can also be strained if the UCL becomes stretched and the muscles becomes the primary stabilizer of the joint.
Thus a strain of the flexor can have 2 causes the primary strain is fairly easy to rehab. The secondary strain isn't the big issue. It's getting the ligament to heal.