Inferno seems to be very comparable to Riot.js, only Riot is more mature, has event delegation and a router, and is only 9.5k vs Inferno's 7k. And riot has an actual web page.
Nowhere does the inferno docs say only people who already use react should use it. As a library for creating "modern user interfaces", it is in direct competition with riot.
I am sure riot's performance is not bad many real world apps. but the devs would certainly prefer the faster one so they won't run into problems later.