Did Jacob really believe that Esau was a lord?
According to Ibn Ezra, it was merely a polite form of address showing respect and courtesy, likely to appeal to Esau’s vanity.
Ramban thinks that a younger brother should always acknowledge an older brother’s seniority.
In the particular circumstances of these two brothers this seems especially appropriate since their parting, all those years before, was largely caused by their relative status in the family: Esau had – rashly – sold the birthright to Jacob and soon afterwards had lost his blessing to his brother.
If Jacob now sought to be at peace with Esau, calling him “my lord” was the first stage in their reconciliation.