Then I totally disagree with that approach from instructors as I feel it is misleading to other drivers. If you are sat waiting to join the roundabout in the left lane but indicating right, it looks to me as if you have got yourself in the wrong lane and intend on going right and need to change lanes. Likewise if you join the roundabout in the left then start indicating right, it looks as if you need to change lanes.
I will come across this if I ever persuade "The Boss" to let me get the motorbike I hanker after if they teach it on CBT.
I was taught that unless the left indicator was on the vehicle was not leaving the roundabout, also that indicating to leave the roundabout was mandatory but to join wasn't. No indicators meant assume nothing and wait for the vehicle to pass.
To be honest I don't believe any indicators I see these days anyway.