I found Sean Carroll’s article on downward causation interesting. One of the things I noticed is that he gives 3 possible definitions for ‘reductionism.’
The claim of reductionism is, depending on who you talk to, that the lower-level description is either “always more complete,” or “capable of deriving the higher-level descriptions,” or “the right way to think about things.”
The last definition is the one Deutsch uses in his books. Deutsch acknowledges in his books that the second definition is actually correct in principle — but not in practice. It is not clear if he’d object to the first definition.
It’s always good to remember that Deutsch uses specialized definitions and that when talking to other people they may have a different concept in mind — perhaps even one that is correct (as in the first definition.)