Where did you find this info? I've searched for it on sjscycles web but couldn't find it.
Have you looked at the Audax 853? (
http://www.thorncycles.co.uk/audax853.html). The smallest size (495S) has a virtual top tube lenght of only 500mm and it's designed for 700c wheels. But I imagine it's quite more expensive than the 531C you've seen, Reynolds 853 is top class!
I'm only 5ft 1", I asked about the Audax 853 to Thorn and they told me that the smallest size should suit me fine.
I own a Raven Adventure Tour from 2004. The top tube is only 490mm, I use flat bars but some times I wish I had drops... so the Audax shouldn't be a problem.
Haven't decided yet for the Audax, I'm now so used to Rohloff! A Mercuy with drops and light wheels is my dreamt bike... but it's so expensive!