> What I don't understand though is his obsession with spaced repetition.

I think Matuschak wants to be able to recall particular things ~forever, and spaced repetition is the best method known for that.

(Nit: Your sentence begins as a genuine expression of puzzlement rather than an outright criticism, but then you refer to Matuschak's "obsession", which pre-judges the issue. It would be more consistent to refer to his "emphasis on" or "advocacy of".)