Disclaimer: I've not recently read any discussions of this topic. And everything I say is my own opinion.

SUMMARY
The syntax of Python's string join operator are a consequence of Python's core design decisions and coding principles, together with the semantics of join. I'll explain this in a series of posts.

Why str.join? Wouldn't list.join be better?
===============================
Python variables don't have types. Python objects do have types. When writing Python, don't test for the type of an object, unless you have to. Instead, test to see if the object has the methods you need.

Eg help(dict.update)
update(...)
    D.update([E, ]**F) -> None.  Update D from dict/iterable E and F.
    If E is present and has a .keys() method, then does:  for k in E: D[k] = E[k]
    If E is present and lacks a .keys() method, then does:  for k, v in E: D[k] = v
    In either case, this is followed by: for k in F:  D[k] = F[k]

Now for the semantics of string join. By Python principles
    From a str and an iterable
    The string join operator
     Produces a str

You question amounts to this. Should the signature be
    (str, iterable) -> str    # Informal type signature of join.
or should it be
    (iterable, str) -> str

At first glance, there's no way of choosing. But in Python we prefer
    aaa.join(bbb)
to
    from somewhere import join
    join(aaa, bbb)

So the question now becomes: Choose between
    str.join(iterable)
    iterable.join(str)

There is some sense in having list.join. But in Python, we can't join the elements of a list without getting an iterable from the list (unless there's something like very special short-cut-semantics built into Python).

So in Python the choice is between
    str.join(iterable)
    iterable.join(str)

Now str.join looks more attractive. But I said, without thinking ahead, that the syntax of Python's string join operator is a consequence of Python's core design decisions and coding principles, together with the semantics of join.

I'm not quite there yet, there's a gap to fill.  I'll pause for a day or two now, just in case someone else wants to JOIN in the discussion, and perhaps fill the gap.

-- 
Jonathan