From the Planck publications, it is seen that the Hubble constant comes from a fit to the CMB data in a specific model described here :
Within the minimal, six-parameter model the expansion rate
is well determined, independent of the distance ladder. One of
the most striking results of the nominal mission is that the best-
fit Hubble constant H_0= (67+/-1.2)km sec^-1Mpc^-1 , is lower than
that measured using traditional techniques, though in agreement
with that determined by other CMB experiments
It is referred in the newer publication here .
The Planck 2013 analysis showed that the temperature power spectrum from Planck was remarkably consistent with a spatially flat
ΛCDM cosmology specified by six parameters, which we will refer to
as the base ΛCDM model.
The disagreements with other methods are discussed in section 5.4
So a specific cosmological model is used to fit the data.