Pondering on Machinist lore rationalisation

[Fenral]

Or will it just be handwaved jarringly, that the player is a master machinist, yet simultaneously incapable of even the most basic workshop crafting

This. The Warrior of Light is a magical unicorn princess with the ability to be a natural-born anything as soon as s/he picks up the required tool for the job, at the cost of forgetting all but 5 (or 10) of their other acquired skills.

Actually, I suspect it's a combination of the above plus the knowledge inside the usual Soul Crystal. A Soul Crystal that will be of absolutely no use to your armoring, blacksmithing, goldmithing, mammet-fixing endeavors.

The writers seem perfectly content with pretending the WoL is only good at whatever Job s/he's on at the time, so I suspect it'll just be more of the same.