Usually 'hardened wood' is just wood where the air has been replaced with epoxy. The problem is epoxy is expensive and not different enough from a pure plastic.
I think what you're describing is more often referred to as "stabilized wood", and doesn't involve the "densification" process as described by the infographic. I'm not willing to pay for the article to read it, but it seems like this is pretty different from epoxy stabilized wood (which isn't particularly hard).
Stabilized wood is essentially as hard as whatever it's stabilized with. But yeah, the interesting concept is the densification. I dunno about replacing steel, but if soft fast growing woods can be converted into something closer to a hardwood, and the volume lost and extra time still puts it out ahead, that is pretty exciting.