I think it is both... I joined a family in their daily hot sauna and then jumping into the cold lake water in Northern Finland. It was a bit odd as everyone was totally nude, from grandmother to grandchildren. And, as you looked around the lake, so was everyone else. The people were very healthy into old age.
I suspect you’re right.
And no question, everyone from runners to meditators to hynosees to yoga practitioners has practiced and experienced the effects of short and long inhalations and exhalations.
But a study that confounds multiple practices doesn’t show much about any of the practices individually.