If you say that X is extremely possible does that mean that X is unlikely to happen (probably won't), or that X is very likely to happen (probably will)?

I'd never come across anyone using this phrase until Nate Silver did. Is it a US English thing?

I’ve not heard “extremely possible” used like that before, but it is pretty similar to “completely possible” or “entirely possible”, example in a headline: https://www.spectator.co.uk/2018/10/its-entirely-possible-to...

It doesn’t mean that a broken heart will definitely kill you, just that there is definitely a chance that it will kill you.

I could totally imagine Nate trying to emphasise the idea that 20% is a much bigger probability than say 1% by using language like that, but put too much weight on it here leading to confusion.

Read very much