I'm not sure I understand why this would make steam machines useless. The main value of a steam machine, as I see it, is that it allows you to have the convenience of a console in what is essentially a generic gaming PC. That is, it has a controller and a GUI aimed at connecting it to a TV and not using a keyboard or mouse, but it's not a locked-down console. It's just generic hardware that will play all of your PC games, and those games don't become obsolete and unplayable when you upgrade to the next generation.
The article says:
if you can use your Xbox One to play your PC on your TV, then your Xbox One can use Steam and effectively become a Steam Machine.
So what they're saying is, if you have a PC running Steam (which is really all a "Steam Machine" is) and an XBox One, then it's kind of the same as having a Steam Machine. Yes, it is... because you're starting with the scenario where you have a Steam machine. It's like saying, "There's no reason to buy a car, since if I already own a car and I buy a bicycle, it's like owning a car!"
Look, you shouldn't assume that I want an XBox. I can get a PC with better graphics and avoid being locked into Microsoft's ecosystem. I can install game mods, my games don't all go unnecessarily obsolete with every new generation of PC, Steam often has very good sales, Steam doesn't make me pay a monthly subscription for online services, and I can use that PC for other things if I like. To me, the only thing that would want me to buy a console at this point is if there were an exclusive game that I really wanted to play, and I've found that I can live without it. I don't want an XBox, so it doesn't make sense to me to say, "If you buy a Steam machine and an XBox, then it's like having a Steam machine!" I'll just buy a Steam machine, thank you, even if it's not a branded "Steam Machine".