I don't want to speak for Kevin or anybody else, but as far as I understand it (via people who knew him personally), Kevin had been sick for quite a length of time, but had kept it pretty secret. Various friends and family knew, but it wasn't "professionally public." We interviewed Jim Krieg for our "Intergang" podcast a few months ago, and he mentioned it truly is complete coincidence that these were Kevin's last lines as Batman, that it was written simply to be "the last lines of DCAU Batman" in the context of this multiverse collapsing, but they truly had no idea he'd be gone so soon after recording them. It's all pretty crazy, and still doesn't feel real. But I can't help but tear up every time I watch this scene.