True. It's funny to imagine that when Flemeth disappears into that mist/smoke she's really just turning invisible and running away really fast.
There's only thing Flemeth knows better than how to make an entrance, and that's how to make an exit 
So much of Flemeth is theatre, and brilliant theatre at that. She's also really good at avoiding revealing her secrets. Before the Mythal reveal, I was convinced she was Razikale, the Old God of mystery instead for that reason.
I imagine that whoever drank from the well, she knew the instant it happened, and dragon wing is a fast way to travel.
It's notable that we never see her teleport outside of that scene, and even there it's unsure. It uses the same graphic as coming out of stealth if I recall.
Anyway, we're all likely overthinking this. There's always been a division between gameplay and story (like how people often mock the idea of sneaking into some heavily fortified while rogues can essentially go invisible in plain sight). But I don't see Calpernia doing anything you can't make Merrill on an Inquisition mage doing, and both of those are talents on the talent tree.