Yup.... you can take it as a backhanded comment. Or you can take it as a sincere thank you.

I honestly meant to thank the people who made my boys come to me! I think it depends on their bakgrounds. If a cat was well taken care of before it found its way to you, then you can thank the people for loving them until the cats found their way to you. But if the cat was abused and then left because they didn't care about the cats, then slap that sarcastic 'thank you' on them!