Like the op said Spaihts talked about a trilogy from the start so i think this is all bull****, just some angry Lindelof hater. And how the hell did Spaihts version linked the 2 movies? It had a ton of pyramids situated along the ecuator of lv-426 that sent light beams into the sky at the end. If i remember right they didn't find any such thing in Alien or Aliens, Not to mention the life-suport module that crashed next to the derelict with one survivor,that again,haven't been spotted.
I do believe they are throwing ideas at the wall right now. I just hope they go to the engineer homeworld.That is all they have to do and end the story. No 3rd part, don't risk it