He didn't, but most of his retcons, this one almost certainly included, were meant to remedy, not gaps in his factual knowledge, but changes in the type of story he was telling. I don't know at what point in his life he learned that potatoes, tomatoes, and tobacco came from the New World, but the reason they were introduced in his story and then had to be retconned was that the setting had evolved to need a retcon.

There were a lot of other retcons he said he wished he could introduce, but it was too late, and others he was still struggling with at the end of his life. E.g., the map of Middle-earth doesn't match the map of Europe 6,000 years ago, but by the time he decided it should, it was far, far too late.

I suppose you could say his biggest knowledge gap was that when he started a story, he didn't know where it was going to end up.