Yes, I saw that one online, too.
I guess people generally don't expect a cat to think about saving anybody but himself. Cats aren't service animals (although there was a very cute bit of verse titled "Guide Cats for the Blind" several years ago); they don't look for people lost in the mountains.
But one of my cats stepped into the fray on behalf of another of my cats a few years ago.
The beleaguered cat (now passed away) was spunky but elderly. He had been through a number of serious illnesses, so he looked like a likely victim - underweight and without teeth. He could hiss and yowl threateningly, but he just didn't really have a lot of defensive moves.
And the dog was not a vicious one. It was a big, over-friendly visiting puppy who had never met a cat before and wanted to play as if the kitty were another Lab pup.
So this dog is barking and bouncing around, and the poor cat is on a chair hissing and growling and thinking, "What am I going to do now?"
Enter second cat. This one was about five at the time, extremely friendly and talkative, and usually one who would be happy to play with a pup. Second cat jumps into the action, blocks first cat to protect him, hisses, growls, gets out the claws, and goes into a "Go ahead - make my day" mode.
The visiting puppy was so, well, impressed that he stayed very clear of both cats for the rest of the visit! (And then they both teased him.)