I don't think having symbols over NPCs' heads is any more detrimental to immersion than seeing an NPC's name and hitpoints when you run the mouse cursor over them, for example. The problem with symbols, IMO, is that they can be used to dumb down a game (by guiding the player through a specific path). I really doubt that Larian would do that, though.
There needs to be some way (preferably subtle) to draw attention to significant NPCs. Traditionally these NPCs have been dressed more elaborately, or positioned in strategic locations, etc, which is harder to do if you have a lot of unique NPCs that can move around. If the symbols only show up when you get near an NPC, then I think it would be something you could just get used to and ignore (if it is not possible to turn it off).
A ruin at NPCs' feet might fit into the game better, as it could be part of a dragon slayer's abilities (if you can read minds, you should be able to detect when someone nearby is trying to attract your attention). Actually, even if the ? and ! symbols were just changed to ruins, colour coded balls or an aura around an NPC's head, it would be much more immersive. That would still leave the issue of how much guiding / hand-holding is wanted or required, but I'm sure that would be something that was brought up in the design of that system.
In any case, the artwork wasn't done when the video was made, so the symbols may or may not have been intended to be final in style or implementation.
