Interestingly, this is similar to an example quoted in that handbook: shadow circle methods include allying with races considered "uncivilized", such as goblins, orcs or giants, as long it is against civilization.
That would mean that an ideologically consistent evil Druid player character would end up wiping out the grove with Minthara 🧠
Not necessarily, because Minthara also represents civilization. It's not about evil vs. good, but nature vs. civilization. The goal is not the eradication of all civilized life, but rather destroying all civilized habitats (such as cities and villages), and thus enforcing a return to the more natural ways of life, because it'll be the only way of surviving once civilization is gone.
My guess is that the druids in BG3 are hoping locking of the circle will protect them from Minthara (who for all her posturing doesn't have that much magical power), while goblins slaughtering the tieflings is survival of the fittest in a way.
edit: Evil druids can join the group of all other evil characters who are not happy with how the evil plotline is being handled.