  Using next-to-leading order (NLO) perturbative QCD, we calculate the
diffractive contribution to inclusive dijet photoproduction in Pb-Pb
ultraperipheral collisions (UPCs) at the LHC and find that it does not exceed
5% in small-$x_A$ bins in the ATLAS kinematics at $\sqrt{s_{NN}}=5.02$ TeV. Its
smallness is a result of the restricted kinematics ($p_{T1} > 20$ GeV and $x_A
> 0.001$) and the large nuclear suppression of nuclear diffractive parton
distribution functions predicted in the leading twist model of nuclear
shadowing. Thus, in an analysis of new constraints on nuclear parton
distribution functions (PDFs) at small $x_A$ using the LHC data on inclusive
dijet photoproduction in heavy-ion UPCs, one can safely neglect the correction
factor due to the excluded diffractive contribution. At the same time, applying
our framework to proton-proton UPCs at $\sqrt{s_{NN}}=13$ TeV, we find that the
ratio of the diffractive and inclusive cross sections of dijet photoproduction
can reach $20-25$% for $x_p \sim 5 \times 10^{-5}$.
