Any manufacturer will be able to make it, but they will still have to adhere to the minimum standard set.
So if that turns out to be a (possibly fictional info for point making purposes ahead) AMD HD4500 and it's equivalent Nvidia Gforce X350 both with 512MB ram, dual core 2.5Ghz CPU & 2GB DDR3 1333Mhz RAM, it still has to have a GPU of that level or better with a CPU at that level or better with that amount of that type of RAM or better and all games aimed at Steam will aim at those specs at minimum. So I don't really see the problem.
If a manufacturer doesn't meet that minimum spec, then they can't license the name Steam Box. Some manufacturers might aim for the high end user with the Steam Box Platinum that has 16GB RAM w/ an 8core CPU and dual GTX580's that runs every game at full res with updated texture packs at 120fps while others may aim for the low end minimum spec user.
But even with the rumored specs
"Steam Box include a Core i7 CPU, 8GB of RAM, and an NVIDIA GPU" I don't see it not being able to run just about every game at some acceptable level for quite a long time (especially since the games will be designed with that minimum spec in mind). Even the highest of the high end games in 2016 will still run just fine on that machine assuming the GPU matches the rest of the specs. It just won't run in 4k @ 120fps like you might play on your specced out 2016 PC.