His argument assumes that something is either related or it is not, and this is a false dilemma.
Whitehead, like Peirce and Dewey, has a notion of continuity that eschews the binaries of substance-thinking that misinterpreters like Harman and Bryant apply to it. What motivates the view is the realization that if we accept the false dilemma that Harman and substance thought proposes, then change cannot be explained except in question-begging ways, e.g., Harman's "series of instants." Harman also brutalizes Heidegger on time, so it's no surprise that he's committing the same mistake in process philosophy. He's consistently wrong, which makes me think that he flat-out rejects the possibility of continuity, and begs the question.
For those interested, I have posted many times on processional views of substance and time. I have also posted links to primary texts on Peirce that make his arguments for continuity clear.