The screenshot of /game/widest shows the 4 shapes in <.canvas> component.
However, based on the implementation of assign_shapes/1 shown on page 383, there is only one shape in assigns.shapes and that one shape is the board’s shape. Between assign_shapes/1 and render/1 (both on page 383), there probably are contents that “rectify” the simplistic assign_shapes/1 implementation which are missing in B10.
Also, Pento.Game.Board.new/1 returns all shape names for puzzle size widest (on page 374). The screenshot seems to be showing the palette for puzzle size tiny instead of widest ('cos the paragraph just before the screenshot says “Open up your browser, visit /game/widest…”)