The reasons why mirrorless cameras might be considered 'better' than smartphones are pretty clear. The sensors are larger and produce much better image quality and you can swap lenses for ultra-wide or telephoto shots and more. You have much more manual control for advanced shooting techniques, and the larger sensors mean longer focal length lenses which bring shallower depth of field and a much more filmic or cinematic look.
The best mirrorless cameras are easily capable of professional stills and video capable, of a quality that has perhaps eroded the market for the best medium format cameras and even the best cinema cameras. And yet this quality comes at a cost.
As for the humble smartphone, well, there seems to be an assumption that mirrorless cameras are obviously and by definition better in every way than even the best camera phones, and that while a smartphone might be OK for beginners, that anyone serious about their craft should upgrade to a mirrorless camera.
