Ok the example really make the paragraph clearer. Thanks. Why can't this be ["a", "b", "c"]?

It could, but I think that it shouldn't:

If we pass ["a", "b,c"] to the build setting implementation function unprocessed, it is free to turn this into ["a", "b", "c"] and pass it on to consumers via a provider in that form.

But if we pass ["a", "b", "c"] directly, we forcibly remove information that the build setting implementation function wouldn't be able to recover. For example, this would mess up linker flags such as -Wl,--enable-auto-import.

Given that build settings can already do any post-processing they require in Starlark, I would opt for doing as little pre-processing as possible within Bazel.

Thanks for the explanation. I'm not familiar with the linker flags mentioned. Could you elaborate on how having ["a", "b", "c"] would mess them up?

If you have a (hypothetical) Starlark version of the native --copt, then you may want to pass in -Wl,--enable-auto-import as a single argument. But if all repeated flags also split on ,, then this would be impossible: The final list of args would always include two separate args -Wl and --enable-auto-import, which changes semantics (in fact, in the case of this particular flag, this would likely lead to a compiler error).