I actually disagree with this statement. Teams can be falling behind for a variety of reasons and throwing more people at the problem can often cause further issues especially if the domain / technology / code base / system ..... Is complex.
Onboarding people into a team is an incredibly costly exercise, to do it while a team is already struggling will often create a worse problem. You need to be sure that the reason the team is struggling is purely an hour's in the day and not due to other factors.