What Apple says and Apple does can or are two different things. And it applies to any company in the world. Blindly believing them is naive at best.
> I can't really think why anyone would think Apple would lock down macOS
I can think of one: iOS. Locking down the software and hardware can allow Apple to funnel people to their own stores and services. It is not rocket science to figure out that a company is trying to increase their revenue. Right now you have only MacOS as the operating system, Linux may never come and Windows support is anyones guess right now.