Not defending NextJS, I'm pretty out on it myself, but ISG requires a server to run. It pregenerates static content for defined pages and serves that until revalidating. If you've built a fully static bundle, nothing exists that would handle that incremental/revalidating logic.
I understand that ISG requires a server (Node.js runtime) to run it, that's why I want to disable it for my SSG Next.js static site, but I can't figure out how. I just want static hosting like S3+Cloudfront which is much faster.