That seems a bit high. Looking at my site's analytics, when we got 50% of our traffic from HN, we had about 45% mac usage, compared to 28% normally. Obviously, there are a lot of conflating factors but that suggests that 60% of HN click-throughs are mac.

What is also interesting is how Linux usage is quite low on HN compared to our regular base; HN appears to be only about 7% Linux. (The people who can't use iTunes at all).

(Again, this is a very rough conclusion; the percentages can easily be way off).