I thought there was some mention of Raam being the place half giants came from in one of the corebooks, but after some searching I came up empty. I think a lot of that assumption came from the extra characterization we got for the SKs in 4e. For all that edition’s faults it did a solid job expanding on the existing cities and SKs.
The question of which SK made the half giants (or if multiple SKs are responsible) is an entertaining one. I definitely like the idea that Sielba was responsible for the reasons I mention in my earlier post.
A fun thought would be if Dregoth and Abalech-Re were the ones to have made the half giants and had a falling out afterwards. We know she wanted Dregoth’s notes and treasure for herself and it also adds some background for Dregoth’s first forays into creating new races.
Edit: On an off note I’m inclined to agree with the “not always fatal” camp when it comes to the mothers of muls (there’s a fun in-setting curse). If that’s the case it could be that the mutations the pristine tower caused to spread psionics so much among the populations of Athas is also responsible for muls being possible. Remember in the first Dark Sun boxed set we had a table of random mutations PCs and NPCs could have. The setting was even more more Frazetta than it is now and I love it.