Sub-PeV Galactic diffuse gamma-ray emission (GDE, $E > 10^{14}$ eV) was first detected by the Tibet AS$\gamma$ experiment in 2021, ensuring the presence of PeV cosmic-ray accelerators in the Galaxy. On the other hand, in 2023, the Large High Altitude Air Shower Observatory (LHAASO) detected GDE covering an energy range between 10 TeV and 1 PeV. Curiously, the sub-PeV GDE flux measured by LHAASO from the inner Galactic plane region ($15^{\circ} < 𝑙 < 125^{\circ}$ and $|𝑏| < 5^{\circ}$) is lower by a factor of five than that measured by Tibet AS$\gamma$ ($25^{\circ} < 𝑙 < 100^{\circ}$ and $|𝑏| < 5^{\circ}$). To study the discrepancy between the results of the two observatories, the contamination of the Tibet GDE flux from the sub-PeV gamma-ray sources presented in the first LHAASO catalog is estimated, accounting for the source masking scheme used in the Tibet GDE analysis. Our finding is that the source contamination of the Tibet GDE flux is less than 30% in the sub-PeV energy range and cannot explain the discrepancy between the Tibet and LHAASO GDE fluxes. The residual discrepancy likely comes from the GDE flux masked in the LHAASO GDE analysis, supporting the dominance of GDE over the total Galactic sub-PeV gamma-ray emission.