FWIW, this is new only for Switch, other platforms discussed this far back as 2018-19, where there indeed was some sentiment of “summoners cannot randomly summon themselves”.
I may have to double-check with some of the ones I have … I can remember running double-Emperinazara team for a faction team run, maybe I’m getting that mixed up with this.
But as “hag” is not an official parameter (Troop Type or Kingdom), this implies Horned Hag’s summon pool is coded directly into the spell effect itself, similar to other troops with ‘unofficial’ summon types (e.g. Queen Moonclaw).