He may not be the most famous person to work on Deus Ex but he still has quite an impressive list of credentials. Doug Church has worked on not only Deus Ex but also Theif, System Shock, and Tomb Raider: Legend.
Valve, as usual, isn’t commenting on his exact position or what projects he may be working on. As far as I know, Valve is pretty open about everyone having some sort of input into all of their games. Unless he is a design lead on a new project, his influence might not have too much of an effect on Valve’s projects.