PSLDX does not dynamically adjust the leverage between stocks/bonds as a typical risk parity strategy would. Not that this is necessarily bad, this fund has a consistent exposure to a duration trade, buying long term bonds and paying short-term borrowing rate. over the last 40 years or so this has been a fantastic trade, as interest rates dropping both raises the price of bonds, propels higher equity values, and lowers the cost of leverage.

the downside to this particular fund is the extreme turnover in the fixed income component (only suitable for tax-free accounts) and the interest rate risk; the fund could underperform SPY in a world with increasing interest rates (which is where many traders believe we are now)

It does have an impressive record. 12 years is a good start, but not a long enough track record to prove much. I've been investing for nearly 40 years, and have had many with 12 good years go sour.