The explanation didn't make me any happier. Suddenly she's Mythal. How much you wanna bet Bioware had no idea that was going to be the case when they first created her.
Are you offering to let me take your money? Because I'd happily take your money.
Unlike the ME trilogy, in which the devs have confessed that the nature and objective of the Reapers and the underlying lore was made up as it went along, the DA team has a pretty well documented reference to deep lore and pre-existing worldbuilding on things that include the nature of the Fade and magic, the pre-history of Thedas, and the the truth of the Gods. We don't know them, but the Devs have more than once referred to the fact that these things exist in their internal wiki and deep lore.
The nature of Flemeth and what she is has always been a a deliberate mystery since the start. We knew she wasn't a mere mortal, that she had access to magics unlike any other, that she and Morrigan had deep knowledge of the Old God rituals and more. DAO heavily hinted that her nature was unique, and DA2 was explicit about it: multiple companions (especially Fenris) explicitly identified her nature as unique and unknown, denying the possibility of even an abomination.
While plot lines have been maleable in the DA creation process, dropping this idea or incorporating that subplot here, the underlying lore has been pretty stable and not subject to change. The main 'retcons' primarily have surrounded Alistair, to be honest: the gameplay/lore segregation about the necessity of lyrium, and the chronology for him being Fiona's child, both of these apparently being oversights in the implementation. While the DA team has kept us on our toes and adapting to revelations, it's generally been in the form of introducing new facts to what was vague, not changing pre-signaled positions on the nature of various things and the lore.
Flemeth, legendary witch of the wilds, a seemingly timeless crone with unique knowledge and magics that defy reasonable explanation, being a host for the spirit of a long-missing elven goddess? Yes, I can believe that.