An (unfairly short) tl;dr could be that not only the memory subsystem itself can be used as a side-channel or leak, but also speculative and OoO execution itself. Speculative execution in this instance also does not consider (likely for performance reasons) certain dependencies (such as between a load setting an exception and a subsequent instruction depending on the load). While the result is still correct after settling, this introduces timing deltas.

This basically adds another set of tools to the architectural-level attack toolbox. From reading this I expect we'll see some interesting developments in the future.