He did. He said it only works on 2x speed boosts. I guess it's because the 2x Subs move 2 movement ticks for every 1 tick that the slower sub moves, so it goes like
Base [.] [.] [.] [S]
Base [S] [S] [.] [.]
Base [.] [S] [.] [.]
So the blue S jumps over the red S. That's my guess.