For the ancients, breath was spirit. "Union" refers to "Union with God". This idea was not exclusive to the Indian culture. In English, the roots of words like "inspire", "expire", "respire", "perspire" relate to old root words for spirit and breath.
Modern Yoga inherits the ideas and methods of classical Tantra, though often in a degenerated or distorted way. That is why you have teachers ascribing meanings like that. It means there is a semantic ahift going on, but don't confuse it with it's historical roots.
I expect Chinese martial arts to go into a similar decline. Many of the fighting arts come from either a Buddhist or a Taoist neigong lineage. For example, there is a reason to work on horse stande for a while, but it is non-obvious.
My own teacher in the US was interested in teaching a classical form in a modern way: forms and fighting methods. He was very good at decoding the applications from a form in a flexible way. I ran through the entire cirriculum in about a year. Then I stopped practicing regularly.
It wasn't until years later that I found a neigong method that works with the art. It's something I am undertaking by myself, just myself, my body, and my internal energy. It took two yeara to just get through the preliminaries and shed enough of emotional crap to start integrating things into the movement. Once I started doing thaf, stuff like training in horse stance like the classical teaching method, makes more sense. It's pretty amazing stuff, and even though I am touching the tip of the iceburg, I do feel sad. There are not many who will want to invest the time in this. The payoff takes too long.
That's ok. All things come and go.