Trump is a moron, for many reasons and not just this idiocy.
Yes the agreement was about transferring money from richer to poorer countries. Now why was that?
The West has burnt enormous amounts of fossil fuels to get us to the point we are now economically. Third world countries would also have to also do that to get to where we already are. However with our help (money) they could invest in expensive alternatives to the simple dig and burn methods of energy productions. This is important because to get these countries to our levels of economy and industry would result in the ruination of the planet's climate.
There are 3 choices here:
1 - Invest in these countries to save our world (and maybe encourage its citizens to stay at home).
2 - Allow these countries to advance their industry and economy as we did in previous centuries.
3 - Prevent these countries from advancing and keep them in the 3 world (plus I guess keep complaining about how their citizens want to move to our countries).