No, by hardwood, I mean deciduous trees, like Oak, Elm, Hickory, and yes, Cottonwood. most coniferous/softwood woods are more brittle and splintery than hardwoods, but stiffer and work better for structural uses.
I've suggested Cottonwood sometimes, that or Elm are so resistant to splitting and splintering that they are good for utility uses. Doug Fir is the WORST for splintering and sticking your finger or ankle with sharp spikes. Go with green treated SYP over Doug fir every day of the year.
Cotton wood is not oak or apitong, you need to cut it 50% thicker than the SYP you took off, when it dries it will still be lighter than the old planks. The toughness and shock resistance makes it great for scaffolding planks, etc.