"Strength" in normal engineering terms means the load it takes to break something... which is different from "stiffness" which is more of a concern for acoustics.
From what I have been able to find, bamboo is about twice as dense as spruce, and about twice the modulus, or stiffness. That follows the rough rule that modulus is proportional to density. For soundboards, density (or the inverse of it) becomes a more powerful factor, with lower density usually giving a higher figure of merit (radiation ratio).
There is one glaring error in there that jumped out at me, making me a bit suspicious of the whole paper. In table 2.2, it shows the modulus of steel is only ~5% higher than bamboo. That is off by an order of magnitude. Interestingly, the modulus/density ratio of steel is approximately the same as spruce and bamboo... but since steel is so much denser, the modulus is much higher. And it makes a crappy soundboard, where bending stiffness/weight is important. (resonators in dobros are designed differently).