So school kinda lied to you. Herbivores and carnivores exist on a spectrum. The extremes are known as "obligates". So an animal that lacks the ability to eat plants entirely is known as an "obligate carnivore." Otherwise, the rest of animals are somewhere spread across the spectrum. Animals in this region are commonly known to eat what's easily available with just a preference toward one end or the other.

A lot of herbivores will eat meat when starving or stressed. Or just if the happen upon easily accessible meat. Most animals aren't confined to only one food source. To be a omnivore, they need to regularly seek out, consume, and digest both plant and animal parts. Wolves eat grass sometimes, but they can't digest it.

This study is crazy because there are multiple individuals showing this behavior. Not to mention they are actively hunting the voles. Indicating that this behavior wasn't opportunistic. Which is really cool! Though, a squirrel with the appetite of a shrew sounds terrifying.

Pretty much. I havent read past the abstract, in fairness.

actually is an opportunistic omnivore and more flexible in its diet than previously assumed.

Copy\paste this for any organism capable of digesting another in order to survive. The hunting behavior is fascinating to see in squirrels, though.

Yes. In this case the difference is the squirrels are hunting, killing, and consuming voles.

Yes, they are omnivores.

Rodents don't need to be pushed to starvation, but nuts and plants and small insects are safer to forage for compared to hunting something large enough to fight back. So if they can easily get what they need from available plant sources, they will choose that first.