Their model is DRM, but they have no vendor lock-in (in theory, at least). You can use a Valve product (or non-valve product) and never use Steam, or use Steam on any of the 3 major OS's. You can't say the same for an apple product.

I say in theory because yes: Steam in reality has a monopoly in the PC gaming market. And while many games are on multiple stores, odds are many games will only be on Steam. That's a more invisible hand form of a walled garden, but there's definitely an argument to be made there.