en.wikipedia.org/wiki/Sarah_Churchill,_Duchess_of_Marlborough

The claim is too absolute. The official Windsor Great Park history page gives a chronological list of Rangers and includes multiple women besides Sarah Churchill:

• Sarah, The Duchess of Marlborough (1702–1744)
• Henrietta, Countess of Godolphin (1709–1733)
• Mary, Duchess of Montagu (1709–1751)
• Queen Victoria (1837–1841)

Because the official list names other female holders of the office, Sarah Churchill was not the only woman ever to be Ranger of Windsor Great Park.

This statement is too broad. The Database of Court Officers 1660–1837 records other female holders of the title Keeper of the Privy Purse besides Sarah Churchill.

Examples include:

• In the Household of Augusta, the office list shows Lady Hamilton and later the Countess of Middlesex under Keeper of the Privy Purse 1736–1772.
• In the Household of Caroline of Brunswick, the office list shows the Countess of Carnarvon under [Keeper of the] Privy Purse.

Those records mean Sarah Churchill was not one of only two women ever to hold that title. If the article intended a narrower claim (for example, only in a reigning sovereign’s own household), it does not say so; as written, the statement is inaccurate.

The date and institution are being conflated.

Official Commonwealth history says:

• In 1959, Queen Elizabeth II placed Marlborough House at the disposal of the British government as a Commonwealth centre.
• The Commonwealth Secretariat was then established in 1965, and it was housed in Marlborough House.

So Marlborough House did not become the Commonwealth Secretariat in 1959. The correct distinction is that it became a Commonwealth centre in 1959, and the Secretariat’s headquarters in 1965.