Question: Would bolusing a hypothermic ROSC be considered active rewarming?
No. Room air is 23 degrees Celsius and as such, so is your IV fluid. A patient would have to be massively hypothermic to have room air temperature saline be considered an active rewarming strategy.
Even when saline is warmed to 43 degrees Celsius, the warmth dissipates so rapidly that this is not an effective means of active core rewarming in profoundly hypothermic patients. These patients require an invasive strategy such as cardiopulmonary bypass, peritoneal or thoracic lavage and this is one (but not all) of the reasons for the one shock strategy and rapid scene time/early transport.