As the saying goes, is it even theft if you don't own the device? (If you can't do whatever you wish.)

Yes, Apple rents me some very powerful hardware that allows me to make a living.

Someone depriving me of it is theft.

If we want to split hairs, technically it’s robbery, which is more serious than theft. In the UK for example, the maximum sentence for robbery is life imprisonment.

That is a particularly idiotic saying.