Highly doubt this story as I don't see what BMW gains.
1) BMW has a dealer network. I'm not privy to the franchise agreement but I suspect it says it agrees to give a dealer allocation to all car products. Selling only or predominantly to Apple is a direct threat to dealerships. Que the law suits and ill-will creation. BMW still has other models it depends on dealers to sell. An all-electric fleet in 5-10 years is wishful thinking.
2) Except for the disastrous Moto ROKR, Apple has never directly collaborated with another company to create a co-branded product. The reason is because that experiment was a disaster. (Carplay isn't co-branded, it's an Apple product that is licensed with strict terms but also Apple techs to advise.)
3) BMW's are already highly s/w reliant so it's not like BMW needs help in this area. It's s/w controls not just your phone but also the hard or softness of the suspension, steering tightness, how hot the seats can get, tire pressure monitoring, emissions monitoring, etc.
4) The BMW brand is already well regarding and BMW cars among the most desirable and copied in the world. The i3 is fugly to be sure, but the i8 is gorgeous. So even in design, BMW doesn't need help... maybe a head slap from time to time though.
BMW could use much help in the GUI/user friendly department. And that is where Apple would be a huge benefit for them. But that's as a part's supplier, not a development partner. No different than BMW's relationship with, say, Siemens or ZF.