I have often heard, that adding people to a late project would be ineffective. How true is this?

We are late into a client project, and the codebase is significantly large if I might add. There are opposing arguments in the team.

My manager is all up for getting “new talent”, but I feel anybody who is not significantly senior in their role, would have to be given a primer to the codebase, and wait until they get a hang of it; which would waste consider amount of resources in terms of time.