I did not read too far into the paper, but I get the idea. Yes, it is almost passive indeed! At a range of only 10m I was disappointed that the idea isn't breaking new ground for increased range, as I have an RFID invention that is still waiting for more range. This kind of range is just above the kind of distances one can get with passive tags at UHF and L band frequencies. They communicate in two directions, which is a greater challenge than your example.
At this small range, my concerns about interference are small. It is interference that often drives receiver architectures to greater complexity and greater power consumption. In your paper, the only selectivity in the system is in the antenna and its matching network, so it will trigger from almost any strong signal that comes close. I hope that isn't an issue for you. With a bit more power you can improve a lot on this.
I saw another paper just a day ago that you might be interested in. It discusses the results of experiments to harvest energy from ambient RF power in order to power things like your receiver. They are collecting 60 microwatts and up out of thin air. They use a diode multiplier circuit (charge pump) almost exactly like the one in your paper, only it feeds a power accumulation circuit. I think you need to use something like that. Unfortunately, it often comes down to the antenna, its tradeoff of size vs gain and frequency band. But it can be done. Perhaps this could be used to supplement a rechargeable lithium button cell.
As your paper discusses, there are great benefits to be had from duty-cycling. Can you consider turning your wake-up receiver on and off with a short period, in order to save even more power? Or does it really have to be on every single millisecond?