I've been told that he filled it "only as a joke"
That is false. Don't believe everything you are told.
I filled it because it was a valid order on the market, I had available SP to lease out, and while the rate was low it was a large order with a significant total fee (by contrast, I wouldn't bother with a low-rate order for a tiny amount).
Enjoy your leased SP, and use it well.
Honestly, I didn't believe it was a joke but I do consider it very generous all the same. So thank you again smooth. I'm glad to see that you are still around and appreciate what you've done for the steem community.