So I was pretty sure based on the range, but it took me a long time to find where it says on their site that Flutter is 915MHz ISM band device. So legally that is a ITU region 2-only device, so sorry Kenneth. The ISM band rules are that you have to limit tx power and accept all interference, so the range is a bit of random variable. There are plusses and minuses to any frequency band these days, I guess.
You can get a $12 module at Sparkfun that transmits at +20dBm, which would likely get you 250meters. He must be using spread-spectrum or some other digital modulation technique to get to 1km. The max tx power in that band is spec’ed as an electric field a certain distance from the the antenna (EIRP), not a power level, so modulation matters. I see some folks have transmitters doing +36dBm tx in that band with digital modulation.
It’s a cool idea for remote sensors.