This is consistent with the docstrings of the methods:

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>>> help(Counter.__iadd__)
Help on function __iadd__ in module collections:

__iadd__(self, other)
    Inplace add from another counter, keeping only positive counts.
    
    >>> c = Counter('abbb')
    >>> c += Counter('bcc')
    >>> c
    Counter({'b': 4, 'c': 2, 'a': 1})

>>> help(Counter.update)
Help on function update in module collections:

update(self, iterable=None, /, **kwds)
    Like dict.update() but add counts instead of replacing them.
    
    Source can be an iterable, a dictionary, or another Counter instance.
    
    >>> c = Counter('which')
    >>> c.update('witch')           # add elements from another iterable
    >>> d = Counter('watch')
    >>> c.update(d)                 # add elements from another counter
    >>> c['h']                      # four 'h' in which, witch, and watch
    4
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However, it seems Counter.__iadd__ is not mentioned at all at https://docs.python.org/3/library/collections.html#collections.Counter. I think there could be a doc update.