I'm not a philosopher or theologian, but this just seems wrong to me, at least when taken in the context of the entire encyclical and the history of Catholicism. That "God created humanity in his own image" has always been a central tenant (if not the central tenant) of Christianity and Abrahamic religions generally. So it would seem like anything that makes us "less human", or denies us the full power of our "uniquely human gifts", would by definition be making us "less Christ-like", and my read of the rest of the encyclical seems like this is (generally) Leo's point.
Again, I'm not a theologian, but Pope Leo obviously is, and "tying these ideas back to core Catholic principles" didn't strike me as a problem in this encyclical.