Thies Christophersen never claimed to have known everything about Auschwitz--but it is reasonable to believe that if there had been any mass gassings, he would have certainly known about them.
Not only he would have known about them, everybody with no exception would have known about them as there is no way to keep such a mass extermination secret for such a long time in such a public location and it would have been the "talk of the camp" without a doubt, yet all Auschwitz survivors who talked about the crematoriums during and after the war, seem to remember the gas chambers only "after the fact", which is evidence to the power of post-war propaganda.
This also relates to my earlier proposition that none of the believers here bothered to address, that Auschwitz as an extermination camp would be an absurd choice for a "secret genocide" since it was a humming industrial place the size of a small city with civilian workers, civilian contractors, red cross visits, family visits, etc. Nobody in the right mind would pick Auschwitz as a location to carry out a "secret extermination". Sites like Treblinka and Sobibor would make a lot more sense in that respect, but even for those there is little evidence to suggest the sinister function that Holocaust mythmakers attribute to them.