I think it is useful to let the employee know what are the accepted delays in this company, for future projects. But the boss or the manager should have said that earlier, at the beginning of the project, then the employee knows what to expect.

That's quite clumsy to say it only once the project has been released. Anyway, don't take it too personally, he/she probably just wanted to keep the pressure on you so you keep improving and never stay satisfied with your current pace. That's not very good management, but that's the way it is in most companies.