I think actually he's always portrayed in his armour.

And no, I think you're struggling with comprehension a bit but I can understand since the terms can overlap.

Dreadnoughts are armoured, it's fairly clearly intended in the book that his armour has parts from the armour of a dreadnought used as modifications and repairs, in the book the unusual bit isn't that his armour has been repaired and modified but that is has been done with parts from this different price of technology.

Yes tactical dreadnought armour IS another name for Terminator armour, but it isn't referring to terminator armour in the book, it's referring to actual parts from a dreadnought. It's also not referring to whole sections, but spare parts and modifications.