Does the Rejected Takeoff checklist in a B737 specify a speed that you should USE/NOT USE reverse thrust?
The QRH says "apply reverse thrust up to the maximum consistent with conditions"
In this case conditions did not dictate that reverse was necessary - (i) it was a low speed reject and (ii) given the distance he had to taxi to vacate the runway stopping wasn't an issue. Throttles to idle would suffice and also minimise any possible FOD concerns.