GDP isn't wealth, it is a measure of output over a period of time (and wealth is an amount at a point in time). It is also, generally, quite misleading because a nation can produce a tremendous amount of output, and all of that can be siphoned into an offshore account in Switzerland (KSA is an example, but there are also more subtle examples like Germany).
And measuring wealth is very, very complex. Do you include pensions? How do you value a state pension? Do you include housing? How do you value housing? But if you look at relative net financial wealth (inc. pensions, you are likely going to Google and find something that doesn't include pensions) then the statement is true...the US and Switzerland are way out ahead on net financial assets, and in datasets that include pension wealth then you have Australia up there too (which has one of the most successful pension systems in the world, very strong economy too but they punch above that weight when looking at wealth).
Again though, measuring these kind of things is complex so you have to not only be able to Google a ranking system but understand exactly how the pension system is structured in that country, how is the financial sector structured, how is the tax system structured, etc. The US is consistently at the top by any measure though.