[charrisku]

He's saying that if you looked at the whole history of the US and divided that into 10 equal parts, then there would be a 10% chance of any random observation falling into the last part. Hence, all else equal, we have a 10% chance that we are looking at the US in the last 10% of it's history.

[hamburglar]

This would imply that there is also a 10% chance we are currently looking at the first 10% of the country's existence, which means you have confidently calculated the odds of the USA lasting at least 2350 years at 10% based on absolutely nothing. These probabilities are nonsense.