I don't. Henceforth I don't accept the events in its campaign as belonging to the halo universe. To me it isn't Halo if not made by Bungie or 343.
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It's a Halo game and anyone who denies this is only kidding themselves. It's not up to the likes of you to decide what is and isn't canon, both Bungie and 343i recognise it as such - therefore its canon. No idea why you wouldn't call it a Halo game, it has a better story than both Halo 3 and Reach.