Its absurd and something that someone just made up.
There is an influx of people because the UK fanbase extends over the entire state.
For UofL, a majority of the fanbase lives in the metro area of louisville.
As to the surrounding counties, the city of louisville has long outgrown jefferson county. So including oldham, bullet, etc is not some way to fluff the numbers, its simply what has to be done when talking about a metro area. Louisville out grew the county borders 20+ years ago.
Anyway, as to me getting hung up on it, you used it as a premise for your post. In an effort to understand you, I asked for more info. I didn't just assume it was based on some random comment from sombody who doesnt know what they are talking about, even though thats exactly what it is. (not you, but the person you heard say this)
Ultimately, the very premise is completely ridiculous. Almost every living person in the state, outside of the louisville MSA, would have to travel to lexington to make lexington the biggest city in kentucky for a day.