PATA drives were still common enough ten years ago, maybe not in brand new machines but certainly in active ones and readily available as replacements/upgrades. Using a machine from ~10 years ago for document authoring isn't unusual. A good keyboard screen is more important than the rest of the machine for that sort of job - you don't need something new, fancy, & powerful, and many authors prefer an older machine because they can't run distractions well!

He could have been on an old computer - GRRM, for example, still writes on an MS-DOS machine