All the articles mentioning this figure come from low-key websites rather than top media outlets, so I tend to agree with those that doubt it and point out that it's just a rumour and/or a misunderstanding.
I googled Max Keiser, and his biography does not inspire any trust into the things he says. He's literally famous for working on an official Russian state-sponsored TV channel, so I wouldn't expect him to care about the accuracy of his statements.
If Qatar actually invested $500B into Bitcoin, it would be really interesting to discuss. But it doesn't seem to be the case.