You can clearly tell what she means when you watch the whole clip as she is very serious, and the way she mentions Taylor explains it further.
They don't know each other, they have never met, and they probably will never meet since Ariana was the one who decided to stop mentioning her because SHE was young and couldn't deal with the Lambs and the constant comparisons. She didn't handle that situation well at all.
And as for Mariah, I don't know why she is entertaining this **** these days.