Need help on SAT Writing! Sentence error

blue book p602 #24

(Since some) people are (convinced that) dowsing, a method of finding underground water with a Y-shaped stick, is effective, but others condemn the procedure (as) (mere superstition).

The answer was (Since some) and I understood it. But I felt (as) is also incorrect and should be changed to “is”, because the original sentence is not following the parallelism rule. Following parallelism is better but not necessary?

when it says “others condemn the procedure…” you can’t write “is” because… english?
you condemn something as something else… you can’t condemn something is something

sorry if that did not help at all haha

The original isn’t a complete sentence. Since some people think it’s effective…WHAT?? What happens as a result of that? It’s not “but other people condemn it,” because that’s another dependent clause.

The verb in that dependent clause is “condemn.” There is no parallelism to make – it’s a run-on if you say “people condemn it is superstition.” That doesn’t make sense.

Thank you I understand it now. I think I tried to follow the parallelism thing in wrong way. It helped me a lot.