I did consider the possibility of Instapaper, Readability or other similar apps, but as you say I couldn't imagine they'd be the majority, even on the relatively tech savvy Hacker News.

As an example of alternate HN clients, hckrnews.com had 124 referrals, ihackernews.com had 85, HackerWeb had 77 and PulseWeb had 48. That's a grand total of around 300 out of 13.5k.

I'd imagine Readability and Instapaper to be big but probably only some small multiple of that at best.