Well it was.

Specifically, the second paragraph is fully supported within Halo 2. Cortana says something along the lines of "I don't think they (Regret's forces) expected us to be here." Later in the underwater elevator section Cortana decodes messages from Truth to Regret where Truth says something like, "You haste has threatened our grand design, the culmination of our covenant." Regret's reason for coming to Earth without a full attack fleet makes good sense with just Halo 2 and cleverly makes even better sense once the later books are added into the mix. I like that a lot.

Maybe the better question would be why didn't Regret just turn his fleet around and win a whole lot of power away from Truth by reporting the location of another Human stronghold / homeworld?