  The ultra-low kinetic friction F_k of 2D structurally superlubric interfaces,
connected with the fast motion of the incommensurate moir\'e pattern, is often
invoked for its linear increase with velocity v_0 and area A, but never
seriously addressed and calculated so far. Here we do that, exemplifying with a
twisted graphene layer sliding on top of bulk graphite -- a demonstration case
that could easily be generalized to other systems. Neglecting quantum effects
and assuming a classical Langevin dynamics, we derive friction expressions
valid in two temperature regimes. At low temperatures the nonzero sliding
friction velocity derivative dF_k/dv_0 is shown by Adelman-Doll-Kantorovich
type approximations to be equivalent to that of a bilayer whose substrate is
affected by an analytically derived effective damping parameter, replacing the
semi-infinite substrate. At high temperatures, friction grows proportional to
temperature as analytically required by fluctuation-dissipation. The theory is
validated by non-equilibrium molecular dynamics simulations with different
contact areas, velocities, twist angles and temperatures. Using
6^{\circ}-twisted graphene on Bernal graphite as a prototype we find a shear
stress of measurable magnitude, from 25 kPa at low temperature to 260 kPa at
room temperature, yet only at high sliding velocities such as 100 m/s. However,
it will linearly drop many orders of magnitude below measurable values at
common experimental velocities such as 1 {\mu}m/s, a factor 10^{-8} lower. The
low but not ultra-low "engineering superlubric" friction measured in existing
experiments should therefore be attributed to defects and/or edges, whose
contribution surpasses by far the negligible moir\'e contribution.
