This kind of drinking is entirely possible. Some people have phenomenal capacities for booze, which IMHO can be explained by both built-up tolerance and genetics. From my personal drinking experience and observing others, I do not believe Mckagen or Sansom were exaggerating.
Regarding genetics, I had a hell of a capacity to drink right when I started. Once you added some tolerance on top, I could function after consuming prodigious amounts -- and I was dumb enough to be proud of it. I should have listened when I was told that was a clear warning sign that I was highly susceptible to developing a drinking problem, which I eventually did.