In it, they discuss the engine and it's tools, as well as the possibility of improving on them:
EJ: We probably have under-invested in those tools in the past, to trade-off against shipping products.
How many people are working on updating the source engine and tools right now?
RW: There’s probably more people than ever. We have as many people working on our tools as we have working on a single project. So, about twenty to thirty core people.
It also discusses the workings of Valve's studio, along with the differences to other studios in their methodology. It's part of a series of five interviews with Valve staff on various topics; worth checking out!