It's not plausible for David to have killed Walter, chop off his hand(and Or facial scars-The give away David uploaded to Walter), change clothes, swallow the embryos and run to meet the others.
The facial scar on the cheek is the give away-AND this scar is emphasised in the scene with Daniels when he's mending his face back on the Covenant.
It just like myself at first went over people's heads, it's a subtle but clear hint.