BYD could say no. And try to sell its own car, under its own badge. But compared to toyota, is has almost zero footprint in Europe any many overseas markets. Trying to built an ecosystem for distribution on such a world scale and support takes at least a decade if not two. Getting new users to accept new brands is not easy. It'd take time even if there was no stigma attached to chinese products. And the political situation today is such that some level of stigma is still there. Perhaps not so much quality wise anymore, but definitely on the government level.
So... imagine then relying on Toyota's network for distribution, support, on toyota's brand relationship with its customers and the relationship toyota has with governments around the world. If BYD can get just a few percent of earnings over the cost it sells each such car system to toyota - that's potentially millions of such car systems per year. Within a very short timespan.
All the while BYD can still work on its own car distribution if it so desires. But income generation wise, this kind of a move can mean BYD earns much much more than any other chinese maker that tries to go into european markets alone, and in a shorter timeframe.
Also something to consider, what is the maximum number of complete cars BYD can actually build? I believe they only have one or two auto production plants, so batteries/parts is a bigger part of their business than the actual cars. I think they barely have a fraction of the capacity of Toyota in China alone, let alone worldwide.
Unrelated to the latest news. I was looking into some PHEVs, and the current crop are so unremarkable. I can see how LI-ONE can sell so well despite being the only model in their lineup.
Most PHEV are built from ICE to PHEV, so the main powertrain is based on ICE with all the complexity of transmission, differentials, etc. etc. With it, you have smaller battery/Electric range and thus the extra cost of the EV powertrain is not very economical (Less than 50Km EV range).
Li is basically the only EV-oriented PHEV now that the Chevy Volt is no longer in production.
The question is that whether the ICE powertrain is worth putting in versus more batteries, and in most cases it is probably not, which explains the luxury price tag for the LI-ONE.
