Went and tested and it's a 90 degree cone if anyone was wondering.
E: The way windower and/or the game reports facing and position of models makes measuring it a little iffy. I tested by using the TP return when using reforged relic body, and sometimes the reported angle where overwhelm works would be a little higher or lower, depending on things like model size, position, and heading. I guess if you really cared you could probably safely call it always at least 85 degrees, but I imagine SE probably intends it to be 90.