I'd say that CodeSandbox has a strong focus on extending the existing workflow for developers. That's why we have a VSCode integration, a GitHub integration with a GitHub App that creates a running dev env for every branch/PR, and we make sure that generally all editor features you expect (autocomplete, go to definition, hover info, etc) are available for the languages that we support.
No, we've built our own web editor & iOS code editor. In our v1 editor we did run VSCode in the browser, but that was before code-server was released (in 2018). Even if we wanted to run code-server, that would be impossible as we allow for multiple users to open the same sandbox/branch, which wouldn't fit the model of VSCode server (which is single user per server).
I'd say that CodeSandbox has a strong focus on extending the existing workflow for developers. That's why we have a VSCode integration, a GitHub integration with a GitHub App that creates a running dev env for every branch/PR, and we make sure that generally all editor features you expect (autocomplete, go to definition, hover info, etc) are available for the languages that we support.