Butters whole thing about the mantle just uncapping his emergency strength makes no sense, and its exactly the sort of mistake I'd expect a doctor to make, because while the untapped strength reserve thing is true of most people, its not true at all of people with power/strength training (which doctors often fail to understand the differences of). Harry can't be stronger than world class weightlifters just cause he got his brain rewired (nor does he need supernatural brain rewiring between the progressive strength training post coma rehab and the parkour! habit)

And Jim's clearly either done heavy weightlifting or researched it pretty carefully (His description of what it felt like to lift a car was too perfect), so I think he knows that, even if in a bro-sciency kind of way.