The author seems to use the word "workstation" specifically to refer to machines running a proprietary Unix variant, usually using a specialty (non-x86) CPU architecture.

What you're describing are workstation-class PC-compatibles, not workstations (as the author defines the term).

What the author is really talking about is the death of systems where the hardware (sometimes all the way down to the CPU architecture) and the OS are produced by the same organization as a single, integrated solution targeted at a specific vertical.