Cubic has to go to +infinity on either the right or left. Exponentiating doesn't help with that.

exp(-infinity) = 0

And exp(+infinity) = +infinity, so it has to go to +infinity on one side.

Yes, on the right side, https://statmodeling.stat.columbia.edu/2020/05/16/what-a-dif...

So in addition to log-exp to prevent projections going negative, they clipped the end date at 4 Aug to prevent it from going back up to +inf