When Noah built the boat, they used his cubit. The actual length of that is not preserved in any manner. But since the entire thing was built in accordance with the same measurement base, all parts were proportionally made and they fit together.
It is the claim that we know the length of Noah's arm from his elbow to the tip of his middle finger that I find suspicious. That length was not preserved in any form.
It is fine with me if you want to believe otherwise. One way or another, none of this makes any difference.
One either accepts the account in Genesis or one doesn't. You want to reject it go ahead.