You are missing a point here: string methods were introduced to make switching from plain 8-bit strings to Unicode easier.
Is it the only purpose ? I agree with the OP that using string methods is much nicer and more convenient than having to import separate modules. Especially, it is nice to just type help(str) in the interactive prompt and get the list of supported methods.
Also, these methods are living in the namespace of the supported objects. It feels very natural, and goes hand in hand with Python's object-oriented nature.
(just my 2 cents - I am not arguing for or against the specific case of dedent, by the way)