I venture to suggest that the unfortunate promulgation of the manifest does indicate a scenario that has a higher probability than any other that comes to mind.
On the other hand, in previous acts of aviation terrorism were false or stolen identity documents used?
Surely there are better passports to steal if you want to enter china, than an Austrian and Italian one....?
If the pair were Chinese nationals whose real identities were on a watch list in China, then it would have been clever to choose to impersonate Europeans, even if risky, as their mission did not hinge on getting past immigration in China.
Mickjoebill