That adage is meant for tasks that require a large amount of coordination, e.g. large software projects. This is no such task.

It's like saying you've got a large wall that needs to be scrubbed clean, and having 10 people do it at once would be slower than having one person do it. That is obviously not going to be the case.

There are literally legal firms that employ hundreds of paralegals who do nothing but bulk redaction. They literally have systems to split up huge document bundles and do redaction in parallel.

Well, this task is highly parallelizable. Is it not? Why would there be a central bottleneck that you could not scale out?

You're roughly talking about the trusts popularized as Amdahl's Law [0]. Which, to summarize the nuances in this context, says "It depends on the nature of the task."

[0] https://en.m.wikipedia.org/wiki/Amdahl's_law