I really doubt that the Marruspawn Abomination is related to the Abominations in the Epic Level Handbook, but you are correct that it does explicitly say it so I've adjusted the monster class.
The following text:
"
Abominations are mistakes—the unwanted, unforeseen
offspring of misguided deific concourse. Abortions of
spirit, abominations live on, nurtured by their quasi-deific
powers and the pure, undiluted hate of their forebears and
all naturally formed creatures.
"
Appear both in the entry for Abominations in the Epic Level Handbook and in the Marruspawn Abomination in Sandstorm.
The lore fits very well too.