Interview Question After the Interview?

It's a neutral sign to let you know that you're still under consideration. They mentioned your knowledge and willingness to take on a "tough assignment" but they didn't mention anything about your experience. I think in their minds they thought you were a strong candidate but might be searching for someone with more specific experience with similar assignments. If they don't find someone with more experience with similar "tough" assignments then they might come back to you for an offer. At least they personally offered "please feel free to stay in touch." They didn't close the door on you - so if this job doesn't pan out- it sounds like they'd like to keep you on file for another opportunity there that might be a perfect fit for you.