My understanding is, Jeanette Kahn & Paul Levitz worked out a deal for Kirby to do designs for Kenner, which would allow him to get royalites on. This was many years before Levitz became publisher, and set up a rule where by anyone who created a character, if used in outside media (TV, etc.) could collect royalties on it. They don't get paid for the books, but they do get paid for other stuff.
I've heard, for example, that Steve Ditko got paid for his (and Levitz') STARMAN when the character appeared on THE BRAVE & THE BOLD tv cartoons. And Ditko got ZERO for all the SPIDER-MAN tv shows and feature films. ZERO. Sick business, ain't it?
I guess the SUPER POWERS comic was a spin-off of the work on the toy line. I've seen drawings, and it seems in addition to modified designs for some of his Fourth World figures, like Mantis & Orion, he also designed vehicles and whole playsets, NONE of which, as far as I know, were ever manufactured. Damn shame.
In a further tie-in, Kirby's Fourth World characters appeared on Hanna-Barbera's SUPER-FRIENDS during its last 2 seasons, the final year of which was renamed THE SUPER POWERS TEAM: GALACTIC GUARDIANS to tie in with the toy line. There was, in my view, such a quantum leap improvement in quality over those last 2 seasons, that I have the same reaction to watching them as I do to watching the last 2 seasons of DOCTOR WHO with Sylvester McCoy...
"THIS was no time to cancel the show!!!"