Its not as if members of the chapter are not aware of the state of the thirteenth company, quite the opposite actually. While they are a group of legend to most of the Space Wolves, the head of the chapter do know that they are still about and that they still fight for Russ and the Emperor as loyally as they did ten thousand years before.
When it comes to the thirteenth company and the curse of the wulfen, yeah they all have it but many became the master of it. Only the few who let it take over them were transformed into the wulfen creatures, and though the rest of the Space Wolves lock these former members of the chapter away, the members of the thirteenth make use of them. After all, they are still Space Wolves willing to kill the enemies of mankind are they not.
Were the thirteenth to return, this would likely be one of the biggest clashes they have with the chapter as a whole. However they can't come back, if they did it would raise to much suspicion with the inquisition, a group the Space Wolves have never been on good terms with anyway. (Thats why the fact that the thirteenth are still around and have been fighting in the eye of terror these last ten millenia is not spoken of.)
