How I see the reject scenario is like so.
If the FO is in the process of advancing the thrust levers when ATC says stop, the FO can retard the levers closed whilst the Capt calls STOP and then takes over the thrust levers and the RTO scenario continues as standard.
If the FO is in the process of advancing the thrust levers when ATC says stop, the FO can keep the thrust levers where they are, wait for the Capt to call STOP and takeover the thrust levers, thus initiating the RTO procedure.
It really isn't a black and white scenario.
Common sense says to me for the FO to close the levers.