Whether Strange is an actual authority on magic or not is debatable. He certainly thinks he is, so he is treating this conversation like he is ergo Foster gets ego for days. He (obviously) straight up ignores that colder tone as Foster continues talking.

A quick way to get Strange's attention is to mention the fae. Another quick way is to mention books. Mentioning a book stolen from a fae is enough that all conversation about the Raven King is ignored to talk about this new development.

"What does the book say?" Strange asks, downright interested. "And what is it about?"