From what context I can pick up on, it wasn't really like that. Seemed to be more an opportunity came along that would have meant work for the company, and the company didn't want to do it. Eg. some kind of big collab like MGS or a TV appearance or maybe a concert collab with a big IRL idol group, but would have required a 3d stream... just as examples.
From the context it sounded like Pomu really wanted to do it, so I don't think it would be her turning it down on her end. I don't think Niji were the ones making the offer.
Also worth mentioning that around the same time (mid May), Nina got a bunch of her projects shitcanned, took an extra break because of this, came back and said "instead of getting attached to projects, I am just going to get attached to streaming the games that are on my stream schedule every week", and a month later announced her graduation.