The use of the term 'honour' to refer to an individual's land holdings or estates does date back to the Conquest period, yes. But as spelling of the English language wasn't standardised until the late 18th/early 19th century a number of spellings could be used for a single word, even within the same document. So you might find honour spelled as 'honor', 'honore', 'honour', or any other variation. As Webster's dictionary came after the standardisation of English spelling his 'reforms' and the removal of the u from standard English spelling are technically the alteration to the correct standard spelling of the word, not the word that appears in the OED