The first part says "For all his". We want to ask the question who? the second part starts with Auden and this equates with the first modifier.
For all is an idiom that can be read as 'in spite of', hence the modifier must modify the subject.
One more example:
For all his quirks, Auden was still lovable.
For all is an idiom that can be read as 'in spite of', hence the modifier must modify the subject.
One more example:
For all his quirks, Auden was still lovable.
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