OK. I was wrong. While it is true that van Rijn's team has rescued Falkayn, it is not true that the enemy knows that yet. Just before imprisoning Falkayn, Serendipity Inc. sent two of its six partners in a fast ship to report back to their remote HQ. There is no interstellar equivalent of radio.
Since those two departed, Falkayn has refused to cooperate, been detained and been rescued. With clear evidence that they had Falkayn under brainscrub, van Rijn has compelled the four remaining partners to sell Serendipity with him as broker. Three depart. One remains to settle legal details, then, later, to guide one Solar ship to a rendezvous with the partners' mysterious superiors. So have the three who departed had time to carry word of what has happened back to their superiors? We know only that the X civilization is remote and that the fleet that approaches Beta Crucis would have set off immediately after receiving word from the first two partners who had hurried off even before Falkayn was imprisoned.
I have made the mistake of thinking that, because our side knows something, the other side also knows it.