This is definitely a clever answer.

Because of for chunk in range(0, len(chunks)):, each chunk is an integer. for digit in chunk: is you trying to for-loop over that integer, which makes no sense. I think you want for chunk in chunks:.

You can just write both f str = ... and sc = ... in the same where clause.

If your range is large enough, this recursion exceeds maximum call stack.

> sequenceSum(1,100000,1)
RangeError: Maximum call stack size exceeded
    at sequenceSum (repl:1:15)
    at sequenceSum (repl:5:18)
    at sequenceSum (repl:5:18)
    at sequenceSum (repl:5:18)
    at sequenceSum (repl:5:18)
    at sequenceSum (repl:5:18)
    at sequenceSum (repl:5:18)
    at sequenceSum (repl:5:18)
    at sequenceSum (repl:5:18)
    at sequenceSum (repl:5:18)

I think this is O(n^2), so it's not very efficient for longer strings.

Note that the |> operator comes from Flow package, and is used the same way in Elm and F#. I find it much cleaner and easier to read, especially since you can read it left-to-right instead of right to left as with (.).

How the heck does this work?

This is probably the most efficient, no?