Easy to laugh that people fell for this, but I suppose you have to remember that spaghetti was an exotic continental dish at the time which few British people would ever have eaten. Rice, similar texture etc. is a "plant", so it's not too far-fetched that spaghetti might be. Also it's done with that familiar BBC tone of authority and it wouldn't have occured to many people, who obviously weren't as media-savvy as we all are now, that it was a spoof.