I don't understand what mending has to do with whether a fish is a steelhead or not, plus a spey rod is a mending machine. Whatever.
I grew up chasing steelhead in Wisconsin and Minnesota for 15 years, then moved to Oregon for the next 20 to pursue steelhead. I feel that great lakes fish are definitely steelhead, they are genetically from west coast steelhead stock, so I consider them steelhead. I don't hear arguments about whether striped bass in fresh water lakes are or aren't striped bass, so why do we worry about steelhead?
I will say that ocean and great lakes fish are quite different on the end of the line, but that's a topic for another time.
"Perhaps fishing is, for me, only an excuse to be near rivers." - Roderick Haig-Brown