This is just BS. There might not be other easy/convenient options for people in the US to invest in other economies, but that surely doesn't mean there aren't any options.
One option: gold.
Sure, you can criticize gold as an investment all you want (and I'm not recommending it), but it's definitely an alternative to investing (directly) in the US economy.
I would certainly run the numbers if they didn't say they don't want me as a client.
I'm not saying there's no value for gold at all, I'm saying that it's not an investment in the typical sense. Hold a small percentage of gold as a low-ish correlating asset class to rebalance with? Sounds great. Hold maybe 3-5% of assets in physical gold as a hyper-hedge insurance policy in case the US or world at large collapses? Perhaps a smart move. Holding all one's assets in gold as a perceived investment? No thanks!
The simple fact is that the American economy, and especially American financial instruments absolutely dominate the Global Economy. Consequently, a long term negative growth in the US would, by definition, lead to a Global Economic Downturn.