Tough thing about evidence-based practice. Sometimes you get conflicting results from the “evidence”
Over the last 15 years, I’ve seen eccentric exercise work for my patients with tendonosis conditions (knowing that tendonosis is treated differently than tendonitis but that’s a different topic 
In a nutshell, eccentric activity (elongating muscle contraction) helps to build the tensile strength of the tendon tissue (applying Wolf’s law, although more applicable to bone tissue). In tendonosis, the collegen fibers within the tendon become, in layman’s terms, “misaligned”. This starts to weaken the tendon’s ability to handle/dissapate force. Therefore, by performing simple eccentric activity it “teaches” the tendon to handle load better, thus breaking into this cycle of unhealthy tisse/scarring, etc. Pain diminishes. I also will use some transverse friction massage to help change the unhealthy tissue (followed by some eccentric loading). Patients love it when I dig my thumbs into their patellar tendon
Hope this helps,
Charlie MPT, OCS