Obviously a prank.
Like people would pay $500 to look like a retard in front of people while playing RS.
However, I kind of hope this isn't a prank, I'm not saying I want one because I clearly don't. But anyone that gets 1-99 in this, in such a short period of time, would be really fit.
"Wow, you're in good shape. What did you do?" "I play Runescape, I recently got 99 Strength on RS, same with IRL".
One size clearly cannot fit all, I'd like to see some 7FT tall guy use it. Or some 500 pounder.
However, Jagex did put quite a bit of effort into this prank. I'm sure it got quite a few people.