Biomech Enhancements replace Monkey muscles with super fast cybernetic replacements. This allows Boomerang Throwers to learn the Turbo Charge ability and Super Monkeys to pursue the path of the Robo-Monkey.
The Biomech Enhancements and a wide variety of other Special Buildings are unlocked before the corresponding upgrades. In the case of the Biomech Enhancements building, the Robo-Monkey and the Turbo Charge unlock at Level 20, yet the building unlocks at Level 18.