If there are at least 1000/5=200 floodplanes in the country it's not that surprising; and there are a lot more than 200 floodable areas.

I think you mean 5000 floodplains.

You're right, that was a silly mistake.

Houston had 500-year floods consecutively in 2015, 2016, and 2017. Oh and one in 2001. Must be a terrifically unlucky place.

That or the context in which such events were considered 500-year events has radically shifted in the last 100 and especially the last 50 years.

"500 year flood" just means that in any given year there's a 1/500 chance of it happening. In theory history has nothing to do with it.

In practice it's calculated with models that are tuned with historical data (among other things), so if you fail to notice some important changes and update your models you might be wildly off.

He's saying that they were 1/500 events previously, when the frequency was determined to be 1/500, but conditions have changed (climate, development, etc.) and they are no longer 1/500 events.

Which is kind of the point

Normally caused by non maintenance of flood channels and excessive building - that's what caused two 250 year floods in my village.