it is worth noting that in that episode, the Dominion were expected to be bottlenecked at the Wormhole due to having to come across from the Gamma quadrant. they didn't have a foothold just yet, though there was a small fleet that had snuck through. sending a fleet of ships to bolster interdiction efforts was a low effort, high return gamble. by the time the war started though, the dominion had a sizeable foothold thanks to cardassia, and the romulans had seen that the dominion was willign to use extreme measures like supernova bombs to eliminate threats. getting involved in the war by that time was lots more risk, for much less return.