People seem to have forgotten the lessons of The Mythical Man-Month: it's possible to have hired employees who are negatively impacting the overall software development effort.
Yes, but Brooks was speaking about the effect of bringing new people on to a project. His words, "Adding manpower to a late software project makes it later". However, had those same people been brought on board at the start of the project, they would not have had the same negative affect on the timeline. Certainly some people turn out to be what G. Gordon Schulmeyer called "net negative producing programmers" (NNPPs)[1], but presumably they could have been let go before their impact was a problem.
Could companies be just now laying off the NNPPs they should have let go long ago? Of course, but the track records of these companies hiring the top talent would suggest that NNPPs don't stick around long.
1 http://web.archive.org/web/20011023084845/http://pyxisinc.co...
But if your project is late and you put more people onto it (more people more work done..in theory), it gets even worse.
No you are totally right, it was just about that you then waste even more time to introduce those people to the project then it will save you time.
While I find this a very insightful and important observation -- I'm not sure I see much relevance to current tech layoffs.