Lumi have heads. Lumi are immune to Vorpal decapitation. Therefore, the FAQ answer is blatantly wrong.
I hate to disagree with you Valhalla, but isn't it written somewhere that the specific trumps the general, or something like that?
For example, "if you do something that requires a full round, such as attacking more than once, you can only take a 5-foot step", but there are feats out there that allow you take a full round attack and also move more than five feet.
So the general "only a lack of head" is trumped by the Lumi's "are immune to vorpal decapitation."
Also, when was the FAQ answer written, before or after the publication of the Lumi?