Sorry, talks about "designed to lock in" is a non-falsifiable theory. If you wanted to build a language for the future and even make it open once it's tested and polished, you would do the same thing: gather a small team to work privately without distractions, make it grow in a controlled environment (so you can safely make breaking changes fixable with some migration tools) and when it's mature enough, release it wide open.
I think there's a practical reason for Apple not to lock-in using a programming language. Proprietary tools limit amount of talent being provided to the platform. Really smart brains would spend time either in suboptimal open platforms or choose some other garden (e.g. .NET, Java). It's one thing to lock in consumers into an ecosystem, it's another to detract smart developers by threatening them with lock in.