Could be, but it's awfully hard to say. What would have been cut to make room for embedding work, and what would the impact of cutting it have been? Did we know enough at that time to make a good decision about that?

Given that the embedding API would have had to change dramatically due to e10s pretty soon after 2010 (and maybe again due to Fission), I think 2010 was probably a bad time to promote a stable embedding API.

Embedding doesn't rank high on my list of Mozilla's mistakes.