also wondering this, what's the competitive advantage if they are just buying cells from CATL like anyone else.
but I think the explanation is as simple as they are battery constrained and need more batteries than they can make themselves right now. and yes the CATL cells might not be as good as their proprietary cylindrical cells but it's a tradeoff they think is worth it for the Chinese market.
They are probably going as fast as they can to get their own battery factory tech going, and need a stop gap measure to keep the car making side going. It would not make any sense to invest into the old tech so the only option is to get a temporary ride on someone else's investment.