In the "official" Bluetooth blog, they talk about the BLE pairing mechanism, and how during phase 2 (authentication) the exchange of keys are used to protect against MITM attacks. I don't see how that is possible.
If the attacker creates a device (dx) that sits between BLE device 1 (d1) and BLE device 2 (d2), and simply passes everything through, then as d1 and d2 are exchanging keys, dx is invisible to d1 and d2. After pairing, dx can monitor communications and then, when it decides to attack, it can prevent some packets from getting through, and replay packets it saw previously (all without ever decrypting the packets).
How does BLE pairing, or specifically exchanging keys, have any impact on this?