  Young, low-mass Brown Dwarfs orbiting early-type stars, with low mass ratios
($q\lesssim0.01$), appear intrinsically rare and present a formation dilemma:
could a handful of these objects be the highest mass outcomes of ``planetary"
formation channels (bottom up within a protoplanetary disk), or are they more
representative of the lowest mass ``failed binaries" (formed via disk
fragmentation, or core fragmentation)? Additionally, their orbits can yield
model-independent dynamical masses, and when paired with wide wavelength
coverage and accurate system age estimates, can constrain evolutionary models
in a regime where the models have a wide dispersion depending on initial
conditions. We present new interferometric observations of the
$16\,\mathrm{Myr}$ substellar companion HD~136164~Ab (HIP~75056~Ab) with
VLTI/GRAVITY and an updated orbit fit including proper motion measurements from
the Hipparcos-Gaia Catalogue of Accelerations. We estimate a dynamical mass of
$35\pm10\,\mathrm{M_J}$ ($q\sim0.02$), making HD~136164~Ab the youngest
substellar companion with a dynamical mass estimate. The new mass and newly
constrained orbital eccentricity ($e=0.44\pm0.03$) and separation
($22.5\pm1\,\mathrm{au}$) could indicate that the companion formed via the
low-mass tail of the Initial Mass Function. Our atmospheric fit to the
\texttt{SPHINX} M-dwarf model grid suggests a sub-solar C/O ratio of $0.45$,
and $3\times$ solar metallicity, which could indicate formation in the
circumstellar disk via disk fragmentation. Either way, the revised mass
estimate likely excludes ``bottom-up" formation via core accretion in the
circumstellar disk. HD~136164~Ab joins a select group of young substellar
objects with dynamical mass estimates; epoch astrometry from future
\textit{Gaia} data releases will constrain the dynamical mass of this crucial
object further.
