The S&P 500 is down less than 1% below its all-time high. It’s mathematically impossible to lose 52% following that advice.

I think “with 10% leverage” deserves a lot more than a parenthetical here.

Try it yourself, the game is not randomized. You should receive exactly the same result.

If fully invested in just the S&P 500 with 10x leverage, it seems less mathematically possible to have lost just 52% unless it was an very short time horizon. If fully invested, 5% decrease in the value of the S&P 500, with 10x leverage, would lead to a 50% fund draw down, given no margin call during the 5% S&P draw down. In most years, the S&P experiences at least one short-term draw down off the years peak of at least 10%, which would lead to a complete loss of capital, given that they hadn't already experienced a margin call on the previously 10x levered assets, which would now be levered higher given a loss in the capital base.

If the days selected were purely random, investing in SP500 would be wise (though not with 10x leverage).

In this experiment only 1/3 of days were random, with another third in employment report days and the last third in fed announcement days.

When I did this with -10x SP500 every day the result was a +36% return, which is surely not what you would expect if the trading days were purely random.

Is there a strategy here?

10x leverage is just crazy for individual stock investment. The lesson you should learn from this is not "don't invest SP500" but "don't use 10x leverage".

Get a better broker :-) my SP50 (yes 50, dont ask!) index is doing alwite.

Get a better reading comprehension tutor.