I found this page from DigiKey which says the following about "Just Works" bonding on Bluetooth 4.2+ devices:

"Once the devices exchange their public keys, the non-initiating device will generate a nonce, which is essentially a random seed value, and then use it to generate a confirmation value Cb. It then sends the Cb along with the nonce to the initiating device. At the same time, the initiating device generates its own nonce and sends it to the non-initiating device. The initiating device then uses the non-initiating device’s nonce to generate its own confirmation value Ca which should match Cb. If the confirmation values match, then the connection proceeds.

By virtue of the ECDH key exchange, the Just WorksTM pairing method in LE Secure Connections has substantially more resilience to passive eavesdropping compared to the same method in LE Legacy Connections. However, since this method does not give the user a way to verify the authenticity of the connection, it is still vulnerable to MITM attacks."

Is this version of "Just Works" supported in SDK 6 on the DA14531? The only documentation for "Just Works" that I found talks about the legacy version (this tutorial).