Okay, so, I just got off the phone with a 54 year old fire spinner. She's been spinning since 1959 (when she was 4 years old). I grilled her for information and got confirmations on a lot of things I suspected.

First, yes, "traditional fire knife" dancing started in 1946, just like a lot of those websites agree. According to her father's tales, the poi that he spun were unlit versions from the Maori. So, poi dancing did migrate that way, however, to his knowledge the Maori never lit their poi. Fire poi dancing, as far as she knows, originated on Maui as part of "Mens only" rituals. She knows this because she was one of the first females to attempt it and was severely chastised for it. [Both for playing with fire, and for doing "men things"]. About that time, however, fir poi dancing was becoming public, that is to say people in hawaii were generally allowed to watch. By the time the 60s rolled around, the tourists were getting their first look too, and that's about when it migrated back to the Maori and to the Mainland.

If this is true, then it's entirely possible that the entire rhythmic fire dancing community owes it's origin to Freddie Letuli when he first added fire to his knife from watching a fire eater.
en.wikipedia.org/wiki/Fire_Knife