It is true that steroids should not be stopped abruptly. You should check with your vet for a "step down" procedure so that you can wean the dog off them slowly. Stopping them quickly can be dangerous.
As for the ACL surgery and the test results, even vets are human. I know that we expect them to be infallable but it does happen. They, and their office staff, do make mistakes.
If you are uncomfortable with your vet doing the ACL procedure, you can look around for another vet but you should not postpone the surgery solely for this reason.
Cranial cruciate injuries are painful and even if the dog may not be showing signs of pain now, chances are that he will in the future.
If you CAN manage it medically rather than surgically, that's great but please don't rule out the surgery just because your vet made a mistake.
Let me know if you have more questions.