The thing is, I love watching ScreaM's mechanical play (the way he times his shoulder peeks, wide-peeks etc, the dueling style), not the aim (IMO his aim is nothing too flashy, the crosshair placement does most of the work). S1mple shouldn't really be analyzed too much because he's #1 right now and his flick consistency w/ +3 sens 400DPI is impressive, but something I've noticed is that he out-positions his opponents quite a lot actually, I find it really smart due to 95% of the pros wouldn't even think about doing it.
These are just a few of the thoughts I have on them, but the IGL&smartness thing is probably looked at the wrong way. I couldn't really rank IGLs by how good they are at doing it without actually playing with them, I'm assuming the only way to look at it from spectators POV is just to see the patterns of how good success the IGL's previous lineups had.
I don't really watch XANTARES play, so I don't really have an opinion on him.
Tl;dr - One factor doesn't determine the intelligence of a player.