Uh I got tricked by gen nine, but atm no Fire get it except psychics. Also, caught Pokémon going in the last box you opened is kinda frustrating
Mystical Fire was originally Delphox's signature move after all.
The move originally belonged exclusively to Delphox who is a starter, and thus the move is generally dominantly associated with it. Armarouge gets it rn as it shares a typing with Delphox so flavor wise Armarouge can probably harness similar powers that Delphox can as a Fire/Psychic type.
It had a wide distribution in Gen 8 by virtue of being a TM, in Gen 9 it isn't anymore and Delphox exists again so it's back to being the Delphox move that its fellow Fire/Psychic type Armarouge shares with it due to having the same typing.