It's a very interesting strategy, and the author has done a great job of detailing how it works.
There is one potential dilemma that I can think of that I don't think the author has addressed.
The key to this strategy is rebalancing between the leveraged ETF and the cash to ensure that the 1:2 ratio is maintained. When the ETF goes down, the investment in the ETF needs to be increased, and this happens by contributing to your Roth IRA. This means that some of your contribution limit is "used up" by rebalancing. Once this thing grows to a sufficiently large amount, you could find yourself in a situation where you cannot rebalance to where you want because the contribution limit is not high enough to do so. For example, if you have $300k in your Roth/ETF and $600k in cash outside the Roth, and the market goes down 10%, you would need to move $20k to restore your balance, which is quite a bit higher than the contribution limit.
This would be solved somewhat by keeping some cash in the Roth from years where the market was up. In those years, you sell some of the ETF, but the cash would stay inside the Roth, which could then be used for rebalancing in the future without using up the contribution limit. Because of this, I don't think the dilemma I described is really much of a problem.
That's the only possible downside I could think of. It sounds like a good strategy for people that want to invest much higher than the Roth limits. Thanks for posting it.