She did order it, but the attack came before he left the bridge and since their shields were down and they were not certain if the attackers have left, Steward stayed on the bridge as security precaution.
If he is ordered to continue, he will leave and order more security to the bridge.
As far as I know it is SOP to secure the bridge during red alert which the ship is still under, unless I missed the order to go to amber alert
It's been several minutes after the attack so you can assume that the reduction in the alert happened off camera so to speak. There are always back ups for everyone, as soon as you need to leave the bridge someone will automatically take over your station (NPC).