It's proprietary software masquerading as open-source, for one, and intentionally fosters dependence on its vendor for the plugins that support the most popular languages as well as key features (remote editing, WSL support, integration with the mainline plugin repos), for one.
They do indeed use a different implementation of these features than the one that ships with VSCode, though they're not the original authors of it. They used to host their version separately here: https://github.com/anysphere/open-remote-ssh
But I guess they've just rolled it into the Cursor repo? Idk, because Cursor itself is proprietary.
There's a similar extension for WSL integration, though I'm not 100% sure if Cursor's implementation of this feature is based on it: https://github.com/jeanp413/open-remote-wsl
You can sideload proprietary VSCode extensions to add them to VSCodium or some downstream fork, but you may also have to patch the extensions themselves because some of them refuse to run if they aren't being used with Microsoft's VSCode: https://github.com/OliverKeefe/vscode-extensions-in-vscodium
