Valve was a completely new company then. They weren't going indie, but Sierra didn't pay them for the remake of Half-Life. In the documentary they talk about financing it by creating Half-Life: Day One.
Chet Falizek, a dev who led L4D and a couple other games at valve talks about this a lot on TikTok, now that he's running an indie studio. He's a cool guy, would fit in on .ml or something for sure.
Usually publishers have multiple products in development simultaneously with varying degrees of investment, the more money invested into a studio to develop a game the more urgent they want it finished.