Cathy argues that the use of algorithm in some contexts permits a new scale of harmful and unaccountable systems that ought to be reigned in.
https://www.penguinrandomhouse.com/books/241363/weapons-of-m...
Algorithms are used by people. An algorithm only allows "harmful and unaccountable systems" if people, as the agents imposing accountability, choose to not hold the people acting by way of the algorithm accountable on the basis of the use of the algorithm, but...that really has nothing to do with the algorithm. If you swapped in a specially-designated ritual sceptre for the algorithm in that sentence (or, perhaps more familiarly, allowed "status as a police officer" to confer both formal immunity from most civil liability and practical immunity from criminal prosecution for most harms done in that role), it functions exactly the same way: what enables harmful and unaccountable systems is when humans choose not to hold other humans accountable for harms, on whatever basis.
"The Unaccountability Machine," based on Mandy's summary in the OP, argues that organizations can become "accountability sinks" which make it impossible for anyone to be held accountable for problems those organizations cause. Put another way (from the perspective of their customers), they eliminate any recourse for problems arising from the organization which ought to in theory be able to address, but can't because of the form and function of the organization.
"Weapons of Math Destruction" argues that the scale of algorithmic systems often means that when harms arise, those harms happen to a lot of people. Cathy argues this scale itself necessitates treating these algorithmic systems differently because of their disproportionate possibility for harm.
Together, you can get big harmful algorithmic systems, able to operate at scale which would be impossible without technology, which exist in organizations that act as accountability sinks. So you get mass harm with no recourse to address it.
This is what I meant by the two pieces being complementary to each other.