Was posted in another forum, so will cross post from there.
He's being general on purpose. He could easily say Moma will remain as Aquaman, but chooses not to even with Aquaman 2 still coming.
For example, The rumor is he is keeping Robbie as Harley Quinn. That would make both Gunn's above tweet true and the rumor that the main DC heroes are done true as well. That tweet could also be saying everyone on the Suicide Squad isnt safe either.
He also isnt saying if he is talking about random fan posts with doom and gloom(mostly snyder fans) OR rumors from sites.
1. We know Superman is out
2. This reads to me like Batfleck is out.
3. Ezra is out even if Gunn wanted him back.
4. Fisher is out because of all the fall out with JL.
5. That leaves Gal and Mamoa? Gal who just got a scene removed from flash and Mamoa who Gunn wont come out and say "Yea he will remain Aquaman". He had no problems saying He not touching Reeve's Batman or Pattinson.
Only one I could see staying is Levi since Peter had a hand in making that. Even that may be gone because Black Adam bombed.