Exactly! For CL (my primary interest) you run them long enough to encounter thermal runaway when the piston expands faster than the cylinder. By starting the heat cycling process rich and progressively leaning out, you control the heat, which allows the piston/cylinder to polish the surfaces, including any stresspoints caused by iron growth.

Although they will run without break-in, by spending the time you may maximize the useful life as well as performance of the engine. I use this time to "get to know" how the engine operates, ie. how much to prime, the best way to start it, etc. I consider bench time well spent.
Some engine manufacturers use methods such as tapered bore, tapered piston, or both, to cut down friction except at the top of the stroke. These all hasten break-in time.
One of the older iron/steel engines (Orwick, I think) had break-in instructions to run it until it seized, let it cool, then run it again. When it ran without seizing it was considered broken-in. BUT, it used a sintered iron piston, not mehanite (hope I spelled those correctly).
Still, there is seldom only one way of doing things. Many have had great success with various methods. Whether you need to break-in an engine is a definite MAYBE.

George