Mount ceiling fan box to underside of joist
The only real reason to cut into the ceiling would be if you needed to bring the wires down from inside the ceiling. Since that isn't the case, I would buy some 3&quot; lag bolts with pan heads (something like this) and drive them through the bead board and into the joist.
The reason for the diagram is they want to emphasize the joist part (#2 in the list). Screwing this into bead board only would be a recipe for disaster.