A big part of writing is getting to "the zone" or "the flow." Many writers use typewriters to ensure that the writing mindset is not polluted by other uses of the same devices. For example, reading news articles on their computer instead of writing, etc. To me, it makes sense for him to use an older computer that is not connected to modern distractions.

It's not just about the choice of software. It's also the feel of the hardware, the way the screen shines differently, the position of the machine in your home.

Some things just can't be replicated using retro-style interfaces on new computers.

It is a question of compartmentalization.