The black hole was manifested???

Yes, but it wouldn't explain time travel through a black hole easily, as even theories aren't supportive on that: you enter a black hole ==> you end up 300 years back safe and sound with your spacecraft not getting a single scratch. Unless we argue that impossibilities could be manifested through imagination - similar to the non-existent jelly fish Beth discovers - and that means e.g. one could imagine becoming resistant to gravity, and then ta da! They fly freely all around like Superman! There are no explanations how limitless the possibilities with imaginations of one who enters the Sphere could be, which is pretty scary in my opinion!

Another explanation is that the time travel itself was manifested by one of the astronauts. Or since they'd all entered the Sphere, each one could have one of these fears/dreams/imagination and manifested it (the same way the current scientists each manifested different things which were leading to their deaths).