Well since it says mixed wood, I would think that a variety of wood is used, so it could be pine, but could be something else too and varies from board to board. I'd just stick with the ones that are actually labeled as pine, or poplar or any other of the chin safe woods.
In case you were wondering the "#2 and better" is referring to the grade of the lumber, #2 is the most common general building/woodworking lumber grade, the better the grade the less defects the wood has. Select grade would have the least amount of defects (knots, holes, cracks, etc), so the prettiest, then comes #1, #2, #3, and so on down.