Core is one of those Microsoft branding words that wound up used everywhere, denotes one thing, but means almost something else entirely. (Not unlike the .NET brand itself in its early years.) In the developer side Core has meant "open source rewrites that can break backward compatibility to move forward".

(Windows has also had several Core-branded things, at least as internal brands that have externally leaked over the last few years or been used to describe Windows Insider initiatives, but there they seem to more actually mean "core" as denoted as pure subset, though it sounds often in those cases they did imply nearly-from-scratch rewrites like the Developer initiatives with that branding.)