It's irrelevant because Rowbotham never assumes a flat Earth. It's pretty simple, really.
Rowbotham assumed that his Bedford Levels experiment was sufficient to prove the earth flat beyond doubt. Subsequent reenactments of the experiment have shown that it isn't.
And other reenactments of the experiments have shown that it is. Given that everybody who's performed this experiment who has found the Earth to be round has arguably had an agenda (to prove to the world that Rowbotham was wrong and the Earth isn't flat), I don't see how the conflicting reenactments can even be taken seriously. Let's remember that according to most FEers the Conspiracy has been afoot for a very long time!
Yeah, but he assumed that he proved one, which he didn't
Rowbotham was a true zetetic, and his book was a true masterpiece of zetetic literature, because Rowbotham assumed nothing in his observations. It was all based on sound zetetic reasoning resting on very firm zetetic principles, beginning with his purely zetetic experiments which proved to him that the Earth is flat.