2 possible situations I can think of, based strictly on 'Among their discoveries was a particular transfer to an account controlled by Mr. Ulbricht from another that had been created in January 2009, during the very earliest days of the bitcoin network, which was set up the previous year.':

- the bitcoin wallet, although said to be associated with SN, has changed ownership since its creation in 2009

- SN purchased something from DPR, perhaps something DPR himself was selling on SR, either a legitimate buy or for some other purposes; SN was just a regular customer in this case

In the meantime, I'm eagerly awaiting their paper. Ron and Shamir have previously co-authored another bitcoin related paper titled 'Quantitative Analysis of the Full Bitcoin Transaction Graph'. There's a very interested overview of it in this Gist: https://gist.github.com/jgarzik/3901921 It appears to have some flaws.

I don't know how accurate this statistic is, but the last Gist comment mentions that 22% of bitcoins are held in reused addresses.