My understanding of the linking with the female changeling was that there was more to their conversation. It's supposed to be beyond the average viewer's understanding. Remember how Odo said, "Trust me" to Kira and Garak? For all we know, he could have made all sorts of promises about the negotiations or something. He could have let her win at tic-tac-toe. Or shown her the Federation's amiability to their enemies when hostilities are over. Or told her the reason the Gamma quadrant wasn't coming to their aid was because very powerful beings (that some people considered gods) were blocking them. Who knows?
Perhaps he had convinced her that the Feds were more fanatical in defending themselves than the Jem'Hadar were in attacking them. Well, that wouldn't have bothered her. She wasn't here to assimilate them, just take over anything left standing afterward.