Then you have been lied to. LH would never bump and pay 2 pax 400 EUR/each to accommodate 2 pax with no confirmed reservation.Most likely your new flight to ATH still had seats for sale when you were rebooked, but at the time of departure the flight was oversold and more pax turned up at the gate than LH expected, with the result that LH had to look for 2 volunteers for VDB.

Whilst bumping the 2 pax from the flight had most likely not been necessary in case you had not been rebooked to the flight earlier in the day, it could also very well have turned out that VDB was not needed because two other pax would not have made their connection at FRA and missed the flight. Normally LH is quite good at overbooking flights without having to pay pax to take a later flight at the end of the day, i.e.doing the math wrong.