This isn't a new concept
Self-directed IRAs have existed since the IRA was established by ERISA in 1974. The IRS has always permitted retirement accounts to invest in real estate, private loans, and other alternatives. IRS Publication 590-A makes this clear. The strategy was never hidden by regulation. It was simply kept quiet by an industry with no financial reason to promote it.
In the early days, checkbook control structures were the domain of wealthy investors with access to specialist tax attorneys. That's no longer the case. The same structures are now accessible at a fraction of the historical cost.
The misconception, corrected
The conventional financial industry has dominated retirement investing long enough that many people assume the Wall Street model is the only legal option. It isn't, and it never was. The IRS permits a wide range of investments inside a retirement account. What limits most investors is not the law; it's the menu offered by their current custodian.