There are a lot of so called betta requirements that get confusing after a while. Tank size and heated tanks just to name a few. A lot of contradicting facts that cant go unnoticed.
I get that Thailand breeders have humid climates and extremely fast turn around time with their fish so no heaters and small containers are understandable.
However, while watching the California Betta Society live stream a few weeks back, one of the breeders here in Cali mentioned that he raises his fish in unheated outdoor ponds. Mind you it gets in the high 60s overnight. And just yesterday a Minnesota member (MN Betta Group) mentioned that his fry are living in outdoor unheated tubs. I just Googled their weather and it's 57deg right now (11am)
A CBS/IBC member (Gianne) keeps her award winning bettas in 30 1g containers. They get biweekly WC's and daily flare sessions. These are fish that have to be as healthy as can be in order to enter and win shows.
The way I see it, whatever works for you.