It looks like it works for previously opened files. Perhaps the author didn't want to load the whole tree into the browser, or maybe its a limitation on webhosted vscode?
I suspect that the author didn't want to eager-load all files because the API they're using is rate-limited -- 60 requests/hr when unauthenticated, or 5000/hr if you generate a unique OAuth token. Even that 5k could get burned through pretty quickly for a larger repo, assuming you have to make at least one request per file.