Can BLE transactions be encrypted without pairing?
This is partially correct. Pairing must occur for the connection to be encrypted the first time. However, if you store the pairing keys (i.e. bond), then disconnect and reconnect, you can use the keys that you have previously stored to encrypt the connection.

From the Bluetooth Core Specification Version 5.0, Vol 3, Part H, Section C.4, Security Re-established using previously distributed LTK.


  Devices may re-establish security using a previously distributed LTK.
  The master device always initiates the encryption procedures, and
  therefore there are two possible sequences: master initiated and slave
  requested.


I hope this helps.