I guess my lack of knowledge in the field shines through. Continuous learning is apparently under active research at the moment, and this blog post about it  is less than two months old, so your intuition was right.
If I were to guess the secret sauce I'd say that a mirror might be involved. Is depth information not worth the trouble for these kinds of classification problems?
You might be right there :)
> Is depth information not worth the trouble for these kinds of classification problems?
Yes, it would be, but there's much more to it than that. Also keep in mind that there are parts that are almost transparent and that no matter what background color you come up with there will be a bunch of lego parts that match it.
Colored strobes may also help separating out different color pieces, although I expect that would be overkill.