Adjacency of non-corner connecting spaces in Escape from Innsmouth
From the Rules Reference, regarding "Adjacent".


  Two spaces are adjacent if they share a border, a door, an impassable border, or a wall.


And from "Movement"


  When an investigator or monster moves a set number of spaces, it moves one space at a time, moving from one space to an adjacent space.


The tile sizes are standardized.  They only come in two sizes.  Square, as in your pier, or rectangular, like the alley.  The drawing of the borders is not done expecting somehow a squished down alley to fill only that part of the open space for A, and it's also not presented as somehow offset to only match A in the app (I've played through this too).  It may, in a different scenario or placement, have a door only bordering A or one of the Pier spaces instead, and in that situation would be different.

In your scenario and drawing however, A, B, and C are all adjacent to each other, and therefore B and C can be moved between directly.