You're probably referring to videos like this[1]. That wikipedia clone has very limited functionality and the interface is raw even for internal use. I took a quick-lap over it, I didn't notice the author writing tests and/or using JS.

Indeed Rails is extremely good for prototyping, but building an actual MVP (let alone a product) takes a lot more than that + there are more stages that have been ommited for very good reasons from that video-tutorial.

[1] https://www.youtube.com/watch?v=9zNouhuKaVs