Vite sets rolldownOptions and rollupOptions to the same object, so this shouldn't be needed.

I just tested quickly and this is because in my reproduction, the whole rolldownOptions object is reassigned. If instead only properties are, it works:

		// doesn't work
      builder.environments.client.config.build.rolldownOptions = {
       input: 'src/client.js',
       output: {
         entryFileNames: '__test__',
       },
      };

		// works
      builder.environments.client.config.build.rolldownOptions.input =
        "src/client.js";
      builder.environments.client.config.build.rolldownOptions.output = {
        entryFileNames: "__test__",
      };

Since the compat code adds rolldownOptions as a getter, it should work even if the whole rolldownOptions object is reassigned.

I unfortunately cannot work on this anymore so I'm going to close it, thanks for the help! At least that's context for anyone working on the linked issue in the future