We already knew as a whole from interviews and the like, that GW breaks up the work load with teams. Its very easy to have one team work on the rules for a line, while another, specialized team, works on the models. Break that down even further to army specializations.
What I don't believe is that he works at Warhammer Community, and gets to work on models. That's two completely different subsets/departments of the company. They have painters and community oriented people on the Website half, while the majority that don't get the spotlight are background workers.
Occasionally they'll pull a sculptor or editor into the ---cast interviews, or to be the subject of an article about there hobby, but they do not cross-pollinate workloads.
Also, the Tau rules would have been intertwined with thw PA:5 rules. So they wouldn't have been Workin on just Tau, but GSC and AM as well. While another team, maybe one sculptor, worked on Shadowsun.