There's a sweet spot about 10,000 - 20,000 kms after purchase, wherein the car's engine is in peak condition.

I've heard the same thing from the SAs that cars are broken-in from the factory... but what this really means is that engines nowadays are assembled with tighter tolerances and are dyno-tested before delivery... which means that unlike older engines, they don't need such extensive break-ins (unless they're high-revving performance models, some of which, like BMW's M5, should never be redlined until properly broken in).

But this doesn't mean that engines are perfect from the factory... an engine's peak performance is attained some 20,000 to 40,000 kilometers after delivery. I've noted in testing that test vehicles with fewer kilometers on the odometer actually make a little less power than "seasoned" testers (this is from instrumented road tests). This observation has been made by various testers like Car and Driver, too... and many owners of new cars report increasing fuel economy as the cars get older.