As i drive trucks for a living i know these pretty wel there is no meaning for the flashing in the real world other than to greet or recognise one another or mabe just to show off your custom rig. genreally if your behind a truck and they start driving closer to the edge of the road and indicate towards the middle this could mean its safe to overtake and is considered good manners.
flashing 4 ways or hazards usually means im slowing down some trucks of the modern era apply the hazards if thert ruck has to brake itself or you have gave the brakes some welly i know the new Actros does and maybe the next gen Scan
if you are overtaking and indicate to come back in front of the driver your over taking if they are experiecend they would usually flash you in you could reply to this for courtesy by either using your hazards or signalling left, Right, Left your basically saying thank you
sometimes you can be courtious not just by using lights
if you are being overetaken donr just keep your foot down to slow the inevitable just slow down a bit and make the overtake a bit more quicker
pretty sure i have seen this also in the comments