From a cursory reading I think they're referring to the idea that an electron traveling at v < c is a superposition of an electron traveling at c and a positron

From a cursory reading I think they're referring to the idea that an electron traveling at v < c is a superposition of an electron traveling at c and a positron traveling at c. Which also is where the zitterbewegung comes from, because the two waves don't "cleanly" overlap, there's a bit of (not-yet-experimentally-verified) interference.