The source reports that the game will feature "huge persistent worlds" using a modified Halo 2 engine. The main campaign will still feature Master Chief and will showcase 4-player co-op. The missions, however, will not be linear, nor will objectives been required. For example, if players are tasked with capturing a weapons depot, but decide to forego the assignment, then they will loose the opportunity to seize the additional weaponry and, in effect, will hand these weapons over to the enemy.
For now, we have to write this off as pure speculation (read: April Fool's gag). But these new details are certainly tantalizing and in line with Bungie's method of revealing a game piece by piece, until the puzzle is complete. Look to E3 for a major announcement.