That's completely false. Apple only started the whole Metal effort because of Mantle, which was the origins of Vulkan. But being Apple, instead of collaborating and ending up in proposing things for Vulkan, they pushed for NIH lock-in.
> Lock-in" means that there are alternatives that you could use, but the vendor does not let you.
Which is exactly the case here. Take some project like Wine for example, that works on implementing Direct3D over Vulkan. That's the alternative they want to use, yet Apple prevents them, forcing them to either waste resources on implementing another path using Metal, or to go suboptimal route of extra translation. Who is to be blamed for it, but Apple?
> Metal might be many things, but lock-in isn't one of them.
It wouldn't be, if Vulkan was available as an alternative. But like you said yourself, lock-in happens when the the vendor does not let you use the alternatives. Who but Apple prevents native Vulkan support from appearing on their systems? So Metal is very clearly lock-in, according to your own statement.