  Tension remains between the observed and modeled properties of substellar
objects, but objects in binary orbits, with known dynamical masses can provide
a way forward. HD 72946 B is a recently imaged brown dwarf companion to the
nearby, solar type star. We achieve $\sim100~\mu\mathrm{as}$ relative
astrometry of HD 72946 B in the K-band using VLTI/GRAVITY, unprecedented for a
benchmark brown dwarf. We fit an ensemble of measurements of the orbit using
orbitize! and derive a strong dynamical mass constraint
$\mathrm{M_B}=69.5\pm0.5~\mathrm{M_{Jup}}$ assuming a strong prior on the host
star mass $\mathrm{M_A}=0.97\pm0.01~\mathrm{M_\odot}$ from an updated stellar
analysis. We fit the spectrum of the companion to a grid of self-consistent
BT-Settl-CIFIST model atmospheres, and perform atmospheric retrievals using
petitRADTRANS. A dynamical mass prior only marginally influences the sampled
distribution on effective temperature, but has a large influence on the surface
gravity and radius, as expected. The dynamical mass alone does not strongly
influence retrieved pressure-temperature or cloud parameters within our current
retrieval setup. Independent of cloud prescription and prior assumptions, we
find agreement within $\pm2\,\sigma$ between the C/O ratio of the host
($0.52\pm0.05)$ and brown dwarf ($0.43$ to $0.63$), as expected from a
molecular cloud collapse formation scenario, but our retrieved metallicities
are implausibly high ($0.6-0.8$) in light of an excellent agreement of the data
with the solar abundance model grid. Future work on our retrieval framework
will seek to resolve this tension. Additional study of low surface-gravity
objects is necessary to assess the influence of a dynamical mass prior on
atmospheric analysis.
