In this case it's because she was fixated on a reflection of my mobile phone on the ceiling 😸 but I know she (and many cats) will do the same when they see anything that excites them such as a bird.
Yes, you did the right thing. First - you didn't walk away from this kitten is distress. You fed her. And found her a home. You most assuredly did the right thing 💞💕💗
Cats pull out the chirp like this when their hunting instinct kicks in while watching their preys, such as birds. In my case she's watching a reflection of my phone on the ceiling lol. According to experts, cats employ this vocalisation "when a bird or insect catches their attention ... The cat becomes riveted to the prey, and starts to chirp, tweet and chatter." 😸