Aww that's great. I agree chihuahuas are so smart and easy to train in my experience too, though they can be a bit stubborn at times.
I litter trained my previous chi Coco and it went so well for a few months, but she eventually rebelled against it. With Lilo I didn't bother with all that and trained her to go outside right away and wow, just like with your pup it was so quick. I trained her to whine or bark when she wants to go outside and that only took a couple of days too.
Both chis I've had also learned the names of all their toys. My family always had dogs while I was growing up and I don't remember any of them doing that. A lot of people I meet seem to think that chihuahuas are stupid, but they're really smart little dogs.