Pure carnivory is uncommon, and its rapid evolution needs a special reason
Six percent of a wolf's diet is plant matter and berries. Here's a cute video of a crocodile eating a pumpkin. Here's a (self-described) review of the top seven vegan dog foods. Also, at least thinking of land mammals, to be a "carnivore" means to tear indiscriminately at the something's innards, and to develop some tolerance for the deteriorating remains of a past kill. As a result, carnivores tend to moonlight as scavengers, and can consume the scats of other animals.
For something to evolve pure carnivory, it means there would have to be some fitness cost to even sampling vegetation, so that even the thought of consuming non-flesh would evolve to become revolting, perhaps through a reworking of the innervation by olfactory neurons expressing particular odorant receptors. This means that the plants are not merely inedible - they are actively toxic to your 'elves' in a way they couldn't evolve to deal with. Or perhaps there is some pathogen that cycles between the elves and plants the way pork tapeworm cycles from pigs to snails.
Becoming facultative bipeds, parabolic ears, clawed hands, strong grip ... these tell a story, where a prey animal (perhaps a small primate) can be heard venturing down into the low branches of a tree, and they stealthily sprint and stand to drag it down and kill it right away. If it moves they can run on two legs to follow. They may be able to follow by climbing into the tree, but their quarry is apparently rather agile also, and this can't be a substantial aspect of their behavior or they would have better fur or tougher hide to deal with a charge into branches.
They likely live in areas with a large number of such trees, and explore large regions before waiting in ambush, accounting for their keen sight and smell and "slightly worse endurance". (Some humans, certainly not me, are the world champions of endurance, and can capture or otherwise make use of horses simply by running after them until the horse wears out: "persistance hunting". So the species you describe still has exceptional endurance by carnivore standards.)