It doesn't prove anything of the sort. And slush funds held by local elected officials don't have anything to do with immense trust funds supposedly held in secret. If Hodges really had any information he would have identified it in the suit and sued the trustee (which he doesn't name) in the suit for release of the funds.
$3.8 Trillion isn't a slush fund, it is more money than the federal government collects in taxes in 3 years.
It does prove there is money we don't know about out there.
The money in your example was held by individuals in private accounts owned by them. Accounts the government didn't know about. That is, some parts of the government didn't know. I imagine Morgenthau did pay taxes on it. He's not stupid.