What exactly does this mean for my admissions chances? Am I "in?"

I’m a very competitive debater who will likely make my way onto the national debate team of the country I’m studying in and have won the Stockholm regionals (I’m American, but I’m in an IB Diploma program in Sweden), and I sent the debate coach at Cornell an email about opportunities they could offer me. I wasn’t expecting much, but I got an extremely positive response: “You sound exactly like the kind of student we are looking for to join our team. I have cced some of my coaching staff…” (appended). I know they can’t explicitly recruit; is it presumptuous to consider this a done deal? My scores, grades, and extracurriculars are all up to or above par for Cornell, so it’s not as if this is making up for any glaring holes in my application.

Is there anything I can do to make sure I take advantage of this in the best way possible?

In theory, Admission is not supposed to be influenced by coaches’ or professors’ recommendations. It is good that you got very positive feedback the way you did. Since everything else is good, work hard on your essay. Not making a bragging list of your accomplishments, show Cornell why they should pick you instead of other students of equal quality. Be honest and be passionate about the topic you choose to write.

It’s not a done deal at all. But you have done what you can to get your application to move into the acceptance pile, and if the stars align, it’ll work out for you.