thought that the fish would never reproduce on its own in the wild. Why they thought that I have no idea and it is kind of silly really lol
Its not that silly really.
DFO tried transplanting Atlantics into multiple systems on Vancouver island back in the day (and if I remember correctly even some Chwk River headwaters).
Most of those never took.
Steelhead are similar in some ways.
Each steelhead run is very unique in its genetics, run timings, where they spawn, what water temps they can tolerate, etc.
More so, the time that they spawn directly effects smolt survival based on when the eggs hatch, food available, etc which is diff in each system.
That is not to say the Steelhead cannot adapt to a new river environment, but generally they would die out before that occurs.
Now with a hatchery on that river system for so long, they may be allowing the fish to adapt, that may be why you are starting to see more "wild fish."
Some other examples of this is why the vedder reds colour so quick, and why it is practically all hatchery enhanced (they don't do well spawning naturally in the Vedder). It is also why FFSBC puts so much research into what strains they can stock into each lake, etc, just because each strain of rainbow is well suited for a particular environment (specific water hardness, pH, temp, etc).