I think the only real similarity between Tatum as a prospect and Giannis as a prospect is the potential for serious length in post-draft growth.
Giannis was incredibly raw (and still has weak shot mechanics) but had phenomenal physicals (height, wingspan, huge hands, great leaping and excellent ball-handling for his size). He had huge upside potential, but also huge risk (What if his body didn't fill out? What if his shot didn't develop?)
Tatum had good physicals, but was far, far more polished in his skill set. Tatum has been playing at the highest levels of basketball available to him at each age since he was 14. Danny has almost certainly been scouting him since his mid-teens.
Tatum, if anything, was a 'safe' pick more akin to Olynyk than Giannis. He's a forward with scoring skills that looked very developed and highly likely to translate.
On the other hand, it's also clear that Tatum has far more up-side than Olynyk had. He's younger and seemingly still growing. And he seems to be more of an 'alpha' in his mentality.
The end-point of what Tatum becomes could be similar to Giannis: Freakishly long, roughly 6' 10" forwards with a variety of scoring skills that are mismatch nightmares and defensive versatility. But they were very different as prospects and will take different paths to get there.