If your cat was diagnosed with a heart murmur as a kitten he had a congenital problem. Congestive heart failure is also a congenital disease. Your cat had this problem from birth.
The neutering procedure uses injectable anesthetics which are usually not cardiotoxic (toxic to the heart). However, any surgery is a stress to the body, and stress could have expiated the development of worsening symptoms, or it could have just been coincidental due to the timing of the surgery. However, it did not cause the disease.
In other words, your cat had a pre-existing heart condition. The cardiomyopathy would have developed at any time, in your cat's case it developed soon after the neutering procedure. In a way this is a lucky find. Many cat's are not diagnosed with cardiomyopathy until they are brought in to the vet as an emergency for hind leg paralysis from a thromboembolism (clot) that lodges in the hind legs. The clot is formed in the flaccid heart and travels in the circulation until it becomes lodged in an area in which it is too large to pass, causing a blockage. Most of these cats with a thromboembolism do not recover.
It is not lucky for your cat to have cardiomyopathy, but it is lucky that it was discovered since your cat can now be placed on heart medications which will hopefully prevent embolis formation. Heart medication can be very helpful, both in prolonging life and giving a good quality of life.
It would be best to have your cat evaluated by a veterinary cardiologist for the best treatment protocol.