There's a stat I think quoted in "Code Complete" by McConnell that says the number of bugs in a system strongly correlates with the number of coders. The conclusion is that as the # of coders goes up, the # of lines of communication between them grows exponentially, and it's the lines of (mis)communication that lead to bugs.

This:

1. explains Brooks' assertion that adding coders to a late project makes it later

2. emphasises the importance in clearly defining interfaces between components, interfaces being the "paths of communication" between the coders of those components.

So your assertion is well founded.