How it's a contradiction? I think mass is detrimental but additional leg muscle at a sub ~2.5 squat helps more than it kills. Meaning - you get a higher force output as a + but you need more effort from the CNS/core/other untrained small muscles to move as a - . I see the CNS power in absolute terms (like everyone of us having a battery in their bodies) that is drained more by the additional mass it has to move around, regardless of it's nature.
Adding muscle cannot be detrimental and useful at the same time because there is only one net effect in the end. This net effect might be the result of independent mechanisms, but what matters is the outcome of a higher, equal or lower VJ.
If you put a bigger and heavier engine in a car, it will have a bigger power production (positive effect) but it will burn more fuel and maybe, due to the higher forces, wear some other components out (negative effect).