Yep, that's exactly my point. If your system can process target render frame N and N+1 at the same time (in different processing units as you say), yes in theory you can have 30 fps and a lag of > 1/30s.But the first processing unit doesn't have to wait for the last step to finish, before it can start work on the next image.
It is parallel processing. I simply don't know how it's done in DIGIC, but in principle it doesn't matter. There may be multicore CPU that runs software off RAM, or off ROM, or it can be hardwired, it doesn't matter.Each processing unit is only present once, but they are all active at the same time. So they can be seen as parallel, as you say. It is just not the kind of parallel one may talk about with more general processing.
There are also other interesting considerations, such as memory buffers for 'processing units'. In order to show 30fps, you need to do exposures 30 times a second. Then if you do parallel processing, you need to double your buffers after readouts.