There are certain perfectionist procedure regimens that some people run themselves through. They will start and run - cool down - run - etc to get the engine's parts to expand and contract to a point where they stabilize the engines parts. They do this with a combination of mixture and throttle settings.

Some people crank up the new engine and start flying them in a normal manner as if the engine was already broken in.

The manual is talking about doing it it the way you did. So another run or two and you'll be ready to go... You don't want to run a ABC engine too rich (as it will run) as it will put undue stress on the connecting rod.

Enjoy,

Jim