Bill Gates was answering in the context his own philanthropic efforts. He, and billionaires like him, certainly would suffer from major opportunity-cost losses by investing in space travel suboptimally.

I wonder whether those experts and resources used on space could do feasible climate engineering...

The only environmental existential risk (apart from the human extended phenotype of course :) ) is climate-based, no?

You can't really make people work on things they aren't interested in though. The experts that are involved in the space industry may not function well in other industries, regardless of imminent doom.

But money spent on space interested experts could be spent on climate interested experts.

We are going in circles. My original point was that the money spent on space travel is negligible compared to other expenses.