If they stream 1080p at 6mpbs, I doubt that streaming 4k at 16mpbs have any sense, probably marketing dictates them to do it. You could get more quality by increasing bitrate without any hardware hassle. Content producers spend much effort and money to create HQ, high-bitrate picture and then you flush it down the toilet scaling to low values. And post production for 4k image is much more compute intensive and much more expensive.

I don't know which surprises me more: "an average speed of 15.6 Mbps would be required to stream its 4K content" or "only 15% to 20% of households have the speeds necessary"

Isn't that an absurdly low bitrate?

I'm pretty sure Super HD 1080p is around 10mbps actually.