Rossmann has only ever said that he no longer feels comfortable using GrapehenOS on his personal device due to Micay’s personal grudge.
Which was a lie. He kept using GrapheneOS for a long time after that. Even though targeted updates aren't possible he left his video up with that disinformation anyway.
Rossmann has never even criticised GrapheneOS itself, only Micay.
He didn't have to. All he needed to do was make people think the lead developer was crazy and that the OS isn't trustworthy. His video has negatively affected GrapheneOS for sure. Still does considering comments like these...
he is an unhinged lunatic
and these.