Engines aren't generic. Companies produce them in a wide variety of physical sizes and power output. Better examples would be spark plugs, shock absorbers, lugnuts and tires.
Battery cells being made by battery manufacturers in huge factories makes a lot os sense. Duplication of this effort by dozens of car companies makes little sense. The car companies might still need facilities to assemble the cells to fit the (non-generic) configuration of the cars they build.
Li ion batteries for automotive use are not remotely generic and I haven't seen anything suggesting they are headed in that direction.
The specifics of battery design, chemistry and manufacturing are what differentiate the companies that will thrive from those that will die or muddle through the ICE to EV transition. Tesla's most significant advantage over all other current or future EV competitors is their ability to produce the highest performance and best cost per KWh batteries possible. China's battery company ecosystem of many smaller players and factories is fine for producing huge numbers of simple, small and low cost EVs for the Chinese market. That same ecosystem is not likely to produce enormous amounts of best in class cells that VW, Daimer, BMW, GM , etc. will need to reach future production goals.
They have taken the single most important EV success factor out of their hands and left it to chance. Hope is not a plan.