Yeah but in CS, the program works or it doesn't. So there's none of that wiggle room. CS degree definitely signals something - a minimum competency that is uncertain in a humanities student.
CS grading is not the same thing as just programs working (and often that's not even the most important part of it), and, in my (obviously limited and potentially not representative, I'll admit, but there's not a lot of structured evidence being presented on any side here), the variability between institutions and individual classes within institutions in CS grading is more alike than distinct from that in any other field.
The scope of the curriculum, OTOH, in CS is probably more consistent between institutions and individual students within institutions than is the case in most non-technical fields, particularly for CS taught within an engineering rather than liberal arts institution (e.g., at UC Davis, the difference between a degree from the College of Engineering and the College of Letters and Science), and, further, while there are certainly kinds of analytical skills that are central to most courses of study in English, its not at all unreasonable to expect that the kind of analytical skills that Google is more concerned about are more likely to be central in CS programs.
But, overall, I think people are generally overplaying this. The stated preference for CS over other degree fields even with a slightly higher GPA in the non-CS field isn't anything like an absolute bar. Its saying that a degree in CS is one signal that is relevant to the Google hiring director that has some weight, not that it overwhelms all other information.
As an aside, though, it is worth Google examining whether this signal is given more weight than it should, especially considering their recently-posted diversity numbers and their stated concern for diversity -- since their overall diversity matches fairly well the diversity of CS graduates but clearly doesn't meet what they want, they might want to consider whether part of their diversity problem is overemphasizing having a degree in CS in their recruitment material and selection processes. If it is overweighted -- and I'm not saying that it is -- that would be a lot easier for Google to fix than many of the other things they are trying to do to address diversity (not that those things aren't valuable to pursue even if addressing the weighting of CS degrees can address some of the problem more easily and in a shorter time frame.)
Full disclosure: I am programmer whose only degree is in Political Science.
SO clearly a CS degree as a bar is going to miss an important demographic (potential hiring population). But a CS bar is also clearly useful in an effort to weed out inappropriate hires (vs one that filters for hiring the best).
This is much easier to notice in the rest of the hiring process than the same sort of faking in a humanities context.