Some users have noted that, in Device Manager, the Bluetooth device listed with the Killer Wireless-AC 1435, 1525, and 1535 show as Bluetooth 4.1 instead of Bluetooth 4.2, even though the Wi-Fi adapter supports Bluetooth 4.2. This is not an error.
The hardware itself does support Bluetooth 4.2. However, Windows 10 does not. You can read here where Microsoft lists its Bluetooth support for Windows 10:
This issue does not apply to the Killer Wireless-AC 1550, which supports Bluetooth 5.0. Windows 10 fully supports Bluetooth 5.0.