This is how you do something when the kindness is only on the surface. You smile insincerely, praise insincerely, apologise insincerely — the act looks warm, but you do not mean it. It describes the way, not the person: she can be a kind person in general and still thank you insincerely this once. You will meet it more in writing and careful speech than in chat; a friend would more often say “She didn’t really mean it.” Same idea as insincere, just turned into an adverb.