Philosophical tidbit #3

In his 1999 essay "A Fresh Look at Zuhandenheit", contained in his collection Towards Speculative Realism (Zer0 Books), Graham Harman argues that in discussing Heidegger's concepts of the ready-to-hand (Zuhandeheit) and present-at-hand (Vorhandenkeit),

[t]he custom is to distinguish between causality and perception, invisibility and visibility. In fact, the only relevant distinction is that beween actuality and relation.

But both of Heidegger's concepts are inextricably bound up with relation. They are mutually exclusive modes of Dasein's relation to the object: the information (knowing that) about the object that we gain when we consider it as present-to-hand is purchased at the expense of the usefulness (knowing how) of the object when ready-to-hand.
I understand that this predominance of relation is precisely what Harman wishes to critique, but on what I've read so far I don't understand quite how his argument develops out of Heidegger's theory - it seems more like a redefinition of the terms involved.