Master Chief's half of the Dawn jumped right before the portal closed out, so he'd be nearer to Earth than anything as far out as the Forerunner home world. And if it was the Forerunner homeworld, the Covenant and UNSC would have already been there/stopped by to say hey. A galaxy infested by flood alla sudden has a ship that just warps in out of nowhere using the very technology that the Forerunners created: If it was their home planet, they'd have been on that thing in a heartbeat. They'd know their own technology, They'd also know their own ships. This thing has one life sign, jumped in etc... I'm hearing klaxons blare and armanents get new life already.
Though it may be of Forerunner design, it's highly unlikely (though not impossible) that it's the Forerunner homeworld. If it was, they'd have been watching the events unfolding in the galaxy and more than likely would have stepped in. After all, "we're their kids."
The idea of it being Marathon was a far stretch that may or may not be true. If it is true, then it deffinately does not fit in with the setting of Marathon: Master Chief is too "new", nowhere near industrious enough. Though not all of you trust Bungie, they have come out and said that the two games are not related, however similar the main characters are. The guy that said they were was an ex-employee.
When cities burn and armies turn and flee in disarray,
Cowards will cry "tis best to fly and fight another day."
But warriors know it, in their marrow when they die and fall:
It is better to have fought and lost then ne'er have fought at all.