Hi bogdan ,
That's correct. Note that video enocding is done on it's own hardware asic, so it wouldn't take too much of other resources except ram.
Difference is that if you run it on OAK, the whole compute is on the OAK, as opposed to using resources on the RPI. I would also guess that RPI isn't powerful enough to do eg. 4k 30FPS, even if not using the RPI for any other compute.