IMHO...
any person that believes modern machining and assembly of engines is "perfect" requiring 'no break in', is sadly missing some accuracy....
yeah, tolerances are tighter, engines are being built better, but there is more to "breaking in" than just making the piston rings seal... LOTS more....
When they strap the bike to the 'dyno' at initial startup, and crank it up.... yes, the pistons begin the sealing process... but this is a quick check of engine integrety, and it' is generally done under a "low loading" on the engine, where it can spin up without damage...
Consider every other piece of "moving/contacting/ metal parts... like cams,the cam caps that retain them in machined recesses', cam/valve/spring contact areas, cam chains and their drive gears, piston rod lower end bearings, and the crankshaft finishes, pieces rubbing together made from various metals.... even the oil pump itself....
ALL of these parts DO require some substantial care in break in, so as to not "overload" a bearing surface by lugging, and hammering it. Spinning the engine is good, and providing the designed film of lubricants of every metal part to burnish them in, while flowing oil thru the passages supplying them, flushes out the micro particulates... and the engine becomes "happy".

it's more than ring sealing by far.
ymmv.