Yeah, but one thing that often gets overlooked when talking about 'non-parallelizable tasks' is that most tasks aren't actually non-parallelizable, rather that they are weakly parallelizable (less than linear speed-up), or they have a large time constant (the 'train-up' time of adding additional resources). If you can identify this and recognize your current position, it can still be possible to accelerate your timelines by adding more people in some cases.
I think the main message that independently working on the same task can be a weak parallelization of random time tasks (independently random depending who is working on it). It simply falls in the category of "less than linear speed-up" parallelization. Since it is a trivial one, you can take it into account most of the time, however it's not too efficient.
