If Valve are going to sink their own developer effort dollars into a modeller because they aren’t getting traction from Blender, it doesn’t make a huge amount of sense taking the GPL option when they don’t have to. As has cropped up in many conversations now, the GPL license applied to Blender makes it difficult (& in some cases, impossible) to create addons linking to proprietary libraries. Given the nature of their business & the software they deal with, being able to link to anything they want without worrying about whether what they’re doing will be compliant with the GPL and seen as such by the FSF (not always the same thing, see initial TiVo communications from them) is a BIG plus.
Frankly, there are two realistic options here. Blender cooperates with Valve enough that the hassle of jumping through GPL hoops is worth it or Valve continues to put their developer resources (money, engineers, and knowledge) toward a solution outside the GPL issues. Not expressing a preference or ultimatum here, just laying down what I see as the practical options on the table. If Valve is putting developers on the table, why fork something that has third-party licensing issues, a counter intuitive interface, and wear the community outrage of going against the Blender vision? If they need to be sticking multiple developers on the job anyway, it makes a lot more sense to create something that doesn’t have those issues whilst still being able to take inspiration from Blender if/when they need to.