There is no such thing as diversification.

If you are Long anything you are at risk. Short is the only true diversification.

The investing mantra goes, In order to lower risk one needs investments that do not correlate to US equities. The theory goes a portfolio with non correlated investments won’t move in the same direction as US equity indices- in truth- when the US enters into a bear market, so does every single other asset class. There is no protection in foreign stocks, currencies, or even commodities- if an equity fund gets hit with a margin call, they will dump currencies or commodities en masse.
There is only one true way to diversify, it’s by owning inverse (short) ETF’s in your portfolio. No 401k or mutual funds hold inverse ETF’s managers don’t even go to cash- so there is zero downside protection in your retirement fund.