.... ummm... How did Homura Subaru end up on the team doing the anthology for this CGDCT anime/manga.. Not compiling, just find it bit random a mangaka that does hentai mangas is working on anthology for something so cute and wholesome....
Much like how many a seiyuu has voice acted in eroge at least once in their career, a fair chunk of manga artists have dabbled their toes in hentai, with many releasing a few R-18 doujinshi before landing that elusive CGDGT serialization. And when you come from a yuri doujinshi background, you tend to have a few yuri friends to invite onto your comic anthologies. Look up pretty much any anthology series on this site (K-On! is a good place to start) and you're guaranteed to find a few authors that have a few r-18 yuri books under their belt. (eg. Kuroba, who writes Magic of Stella and has appeared in anthologies for both K-On! and GochiUsa, is known for a few notable Lucky Star doujins).
(An equivalent example in the west would be how Tyson Hesse got a career in Sonic comics, games, animation, and even the friggin' movie, all from writing a fan comic about Knuckles having "rock-hard fist nipples".)