The argument doesn't make much sense to me. Each government is basically a single decision-maker that already has a very large budget. Just by watching the news, we know these decisions aren't always all that rational (though democratic!). Giving more money to a government doesn't increase diversity of funding sources at all.

Starting another large foundation creates a new source of funding with an independent decision-making process. This increases diversity in sources of funding. (Though not as much as breaking it up into multiple charities.)

Whether it's a net improvement or not depends on whether their decision-making is better than average. Unfortunately, little can be done about charities with poor decision-making (there are no market forces, and corruption is possible). But at least it's independent.