I think you're reading it correctly.

Bear in mind that the transfer from Shinkansen to N'EX at Tokyo Station is quite a hike - you need to go up from the Shinkansen platorm (two stories above ground) to the N'EX platfrom (four basements below ground).

I know my way around Tokyo station - I guess I could do it in 10 minutes. It would take me longer if I'm dragging a heavy suitcase. So I would say that 20 mins is enough.... But Robyng recently did the same transfer and felt that 27 minutes was rather close for comfort for her.

Robyng was taking elevators - If you can use the escalators you will get there more quickly.