^It is safe to say it wasn't a rumour. Akari also said it in a magazine interview. Around that time she felt like she wasn't good enough for them. She lost her confidence and was on a low until she graduated from Eggs. Then UF appraoched her and offered her a place in Up-Front Girls, what UUG was formerly called. If that didn't happened Akari said she would have just retire from being an idol. Sengoku was also going thru the same emotions at the time ( before UFG formation ). She was thinking of quiting but her family told her it would be a waist of all the effort she has put in. Then the UFG offer came and Sengoku took it as a sign for a new begining.
It was during this interview that Sengoku described H!P as a place for "the Gods" akin to mount Olympus for the Olympians.
They were asked if they would consider re-joining H!P if they were offered a chance. Sengoku flatly turned it down. She wants "revenge", to show H!P, to proof that they can do it without H!P. The interviewer then added, "So you want to fight the gods" ( Titans vs Olympians )
Akari on the other hand still dreams to be a member of H!P
"Revenge" is not the only reason why Sengoku doesn't want to return to H!P. Doing UUG, she has tasted freedom. Sengoku says they ( UUG ) have more freedom to do things that would otherwise be restricted in H!P. She doesn't want to loose this.