So this isn’t what happened in another of Grundulum’s videos, showing all damage from a single match with a Doomskull being dealt as True damage:
So as you can see, we’ve got conflicting behaviours (Doomskull included in True damage here, Doomskulls not included in original video).
It also gets multiplied by Godslayer traits etc. (and probably other bonus damage traits?), contrary to:
Agreed that it ignores damage reduction, though, e.g. Eternal Stonehammer.