It isn't a myth about the engine...
There is a very good video on YouTube explaining in detail the engine break in period...and it does recommend not flooring the engine during the initial kms covered in it..
Basically you need to give the various seals and gaskets in the engine time to settle down and smooth out any minor imperfections...But the video goes into alot of depth about this stuff so do check it out...(The channel is Engineering Explained)