Aye this theory has popped up a few times on 40k forums. I think that Its only a picture. The artist may have simply overlooked the fact and it more than likely wasn't intentional. Some sources of background for example state that Horus strangled Sanguinius to death...
Heresy's Background FAQ. (Fluff Project)
I do love how we don't even need CotE to comment anymore, chances are he has already said something intelligent before that can be re-applied to the current situation.
CotE Reviews: Prospero Burns (HH Review)
, Age of Darkness (HH Review)