It's not France. America's alliance with France was predicated upon joint war with the British and it was broken two decades later. Then there was the Quasi War between France and America, which definitely rendered the treaty irrelevant.

America's oldest ally is, bizarrely enough, Morocco. That treaty dates to 1786 and, unlike various treaties with France and the United Kingdom, has never been broken. It also has the distinction of being America's oldest treaty. While the treaty itself doesn't explicitly compel either side to render military assistance, it did tie the two countries together and set the stage for future phases of cooperation. Morocco came down unequivocally on the Union's side during the civil war and, though rather weak compared to the great powers at the time, would assist the Allies during WWI and WWII. Today, Morocco is probably America's closest ally outside of NATO (and definitely America's closest ally in the Islamic world). But that's beyond the scope of this sub.

I'm not even sure I'd say that the UK is America's closest ally. That honor probably goes to Canada. That's not terribly surprising, though.

I suspect that this is one of those cases where the writers over-simplified the statement for dramatic purposes or something like that.

What about Morocco? They recognised the 13 colonies as a sovereign nation in 1777. France may have been the first official ally of the United States, but in terms of international relations generally I think it makes sense to describe Morocco as the oldest friend.

Among the great powers though the US & UK definitely have one of the most long lived and strongest alliances, not just in military but also in trade, intelligence sharing etc. - the so called "special relationship" as termed by Churchill which persisted from WW1 more or less till now.

So maybe "oldest ally" is just a simplification of the unique nature of the relationship?