That's strange. Either the description in the manual is wrong and every edge being shared by 2 triangles isn't the only requirement for a mesh to be closed, or your model isn't actually closed.

How have you created your model, have you exported it from an external modeller? If you have, sometimes the vertices are split by the export process. Then there are actually 2 vertices on the same spot, one conected to triangleA and one to triangleB. Try box-selecting a (seemingly single) vertex(vertices) and welding it. If that doesn't help I don't know.