This article is JimJast rubbish. Just delete it. The concept of curvature (or non-curvature) of spacetime is covered in the main article.
Flat spacetime is such spacetime (or 4-space) that its curvature of space is compensated in direction of time this way that the total curvature of 4-space disappears creating a flat object described by Euclidean (or Minkowski which means the same) spacetime.
Such spacetime has a property of conservation of energy and momentum and so the Big Bang hypothesis in which energy is meant to be created from nothing has to necessarily dispose of this feature and insist that its spacetime is curved and therefore allows creation of energy and momentum from nothing.
It is one of weak points of the Big Bang hypothesis since it is assumed in physics so far that energy can't be created from nothing and therefore the Big Bang is considered by many physicists to be a cargo cult science that Richard Feynman held against relativists.