If I'm not mistaken, I think he has been watching my videos on big stones.  Welcome to the forum. 

To date, the biggest stone I've thrown with a sling was a bit over 3.2 kg (7.2 lb).  It was at the end of my 2021 Pumpkin Challenge video.  That was a two-handed throw though.  I think 5 kg is completely doable.  The biggest concern I would have is just making sure the sling is strong enough to hold it.

I don't think I would want to try a 5 kg projectile one-handed, but two hands actually works quite well once you get use to the difference in how the torso rotates.  Or if someone was able to use a style similar to a hammer throw, some pretty good distance could likely be achieved.  I don't know if I have the coordination for that.  Or the time to learn it.