I'm not a fan of tankless water heaters, but putting that aside, rust can only come from iron or steel. Are you absolutely certain there isn't a steel nipple, coupler, or union in the hot side portion of the plumbing? Not to foist the blame on the plumber, but I would accept the manufactures statement that they do not make their units with steel components. If they did, this would be a huge problem for them because all of their heater would show rust. I would not go so far as to call the plumber a liar, I'm sure if he used a steel component it was accidental, but there has to be some steel in there somewhere.