He is saying that Polymyositis is initially treated with higher doses of prednisone. Polymyositis causes actual muscle damage that isn't prevented with lower doses of prednisone. I doubt the muscle damage is prevented with higher doses of prednisone but higher doses of prednisone alleviates the symptoms.

Doctors never want patients on long term prednisone because of the side effects. There are other treatments available instead of long term prednisone for polymyositis. Additional tests might confirm polymyositis. It is good that your doctor is considering an alternative diagnosis along with alternatives to Prednisone.
https://www.mayoclinic.org/diseases-conditions/polymyositis/diagnosis-treatment/drc-20353212
It is generally believed that PMR will go away in a relatively short period of time. Supposedly long term prednisone is okay because PMR doesn't cause significant damage to muscles and/or bones and goes away eventually.

Polymyositis does damage and doesn't go away and is probably a more serious condition compared to PMR.

I was never diagnosed with polymyositis. However my CK level was chronically elevated. Statin-induced myopathy or corticosteroid-induced myopathy or a combination of both was likely the cause.
https://www.healthline.com/health/what-is-statin-induced-myopathy-or-muscle-pain
or
https://emedicine.medscape.com/article/313842-overview#:~:text=Steroid%20myopathy%20is%20usually%20an,studied%20it%20systemically%20in%201959.
Many conditions mimic PMR and vice versa. Many conditions respond to Prednisone. PMR symptoms aren't unique so further investigation is always warranted.